Brook's first fight as a professional took place in September 2004 with a 6-round points victory over experienced journeyman Peter Buckley at the Don Valley Stadium in Sheffield. He would fight again four more times that year to end 2004 with a record of 5-0 in just four months. Brook went about learning his trade against a host of journeymen before getting the opportunity in June 2008 to fight for the British title having compiled a winning record of 16-0.
The championship fight at London's York Hall saw Kell Brook take on Welshman Barrie Jones for the vacant crown. The fight was not without controversy however as the previous incumbent, Kevin McIntyre, had been stripped of the title after he sustained an injury and had to pull out of his scheduled defence against the Yorkshireman. Brook ended up winning the vacant belt with a 7th round stoppage of Jones who had come in at relatively short notice.
Having won the belt Brook was then ordered to face the former champion and the bout took take place in Glasgow's Kelvin Hall on 14 November 2008. Prior to the fight and referring to the bad blood that has built up between the two fighter's Brook said "Once I knock out McIntyre in style then there will be no question mark over who should be British champion." When the fight finally did happen, Brook put McIntyre down three times in the first round to win by TKO to send the Glasgow crowd home disappointed.
On 30 January 2009 Brook returned to the York Hall to meet former marine Stuart Elwell in a voluntary second defence. The fight again ended quickly when the referee stopped the fight in the 2nd round handing victory to Brook. He claimed the Lonsdale belt outright on 20 July 2009 when making his third defence against Prizefighter winner Michael Lomax. The fight, at the MEN Arena in Manchester, was on the undercard to Amir Khan's world championship victory over Andreas Kotelnik and saw Brook win by stoppage in the third round. After the fight Brook said that he believed he was the best in Britain but couldn't say for sure until he's beaten Chorley's Michael Jennings who he challenged to a match-up.
On 25 September 2009, Brook pipped fellow champion Nathan Cleverly for the prestigious title of Young British boxer of the Year, as voted for by the Boxing Writer's Club. The vote was the closest in the history of the award with Brook claiming the honour by one vote over Cleverly who held both the British and Commonwealth titles at light-heavyweight. In winning the award Brook joined a list of boxers which included the likes of Ken Buchanan, John H Stracey, Barry McGuigan, Nigel Benn, Naseem Hamed, Joe Calzaghe and Amir Khan.
Brook's first fight of 2010 took place in Liverpool on 12 March 2010. The fight, for the WBO Inter-Continental title, pitched Brook against Poland's Krzysztof Bienias, a man last beaten by Junior Witter in 2005 and with a run of seventeen straight wins. Brook collected his second career title when in the sixth round the fight was stopped by the referee following a dominant display by the Yorkshireman. Brook's long awaiting fight against Michael Jennings was due to be next on 3 July 2010 until a rib injury forced the fight to be postponed for a second time. The fight, for Brook's British belt was then scheduled to take place between Jennings and former Commonwealth champion John O'Donnell before O'Donnell pulled out and was then rescheduled between Brook and Jennings. Finally, on 18 September 2010 Brook boxed Michael Jennings at the LG arena in Birmingham. Kell won the fight in round 5 following a bad cut above the eye sustained by Jennings which the referee deemed bad enough to stop the fight. The win doubled as Brooks's fourth defence of the British title and his first defence of the inter-continental belt. On 9 December 2010 Brook made his second defence of the inter-continental title beating Ghana's Phillip Kotey with the referee waving the fight off during the second round. The win ensured that Brook kept his number one ranking with the World Boxing Organisation.
On 8 April 2011 Brook split from promoter Frank Warren and joined the Matchroom Sport boxing stable under Eddie Hearn with Brook saying that he was "really buzzing" to have joined the outfit. Hearn said that he was "excited about adding Kell to our stable" which had also recently signed Jamie Jackson former amateur British light welterweight champion part time technical coach now working in the engineering industry. Speaking of Brook he said "Kell oozes class and has that something special about him – in short he is going to be a superstar." Brook's first contest for the promotional outfit was on 25 June 2011 at the Hillsborough Leisure Centre in Sheffield and resulted in a unanimous 12 round points decision against Lovemore N'dou, claiming the WBA version of the inter-continental title in the process.
On 17 March 2012, Kell Brook took his undefeated run to 27 fights by beating Matthew Hatton, brother of Ricky Hatton, on points in Sheffield. The scores were 119-107, 119-108 and 118-109 all in favour of Brook. In front of nearly 10,000 fans at the Motorpoint Arena, Brook added the IBF International welterweight title to his WBA Inter-Continental crown. Brook floored Hatton in the ninth round but could not knock out his Mancunian opponent. The fight gave record breaking viewing figures for Sky's Fight Night Live programme it was also the most watched fight of the year in Britain. Afterwards he said on the prospect of facing Khan: "Of course I want Khan, I've been banging on about Amir Khan, but I'm going to do my own thing and if that fight happens, it happens, and I'll smash him. I want to be fighting at that top table now with elite fighters. I think I've proved I deserve to be there."
Brook faced American Carson Jones at the Motorpoint Arena in Sheffield on Saturday 7 July 2012. Eddie Hearn's Matchroom Sport promoted the fight with it being billed "Edge of Glory" as both fighters saw this fight as a stepping stone to a shot at a world title. The fight was shown live on Sky Sports 1. The weigh in took place in Sheffield's Winter Gardens on 6 July with Brook weighing in at 10 st 6 lb 10 oz (60.16 kg) and Jones weighing slightly less at 10 st 6 lb 6 oz (60.04 kg). Brook dominated the early rounds moving well and connecting regularly with nice two and three punch combinations however as the fight went on he began to tire and Jones was able to land shots of his own. Brook's nose was broken in the 8th round which made it yet more difficult for him to come back into the fight. He was almost floored in the final round but was able to hold on until the bell. Brook did enough in the early rounds to win the fight by majority decision, as the judge's scorecards read 114-114, 116-113 and 115-113 in Brook's favour. Brook was taken to Hospital after the fight due to blood loss from his broken nose.
In August 2012 Matchroom Sport announced that Brook would face Argentinian veteran Hector Saldivia at the Motorpoint Arena in Sheffield on 20 October 2012. The winner would then become the mandatory challenger for the IBF welterweight world title held at the time by Randall Bailey. At a packed weigh in the day before the fight at Sheffield's Meadowhall Centre both fighters weighed in under the 147 lbs limit. Brook started strongly knocking Saldivia down midway through the first round. The second round was very similar with Saldivia unable to deal with Brook's superior accuracy and power. In the third Brook landed a solid well timed straight which sent the Argentinian to the floor again. This time he struggled to get up as his legs wobbled beneath him and the referee stopped the contest in the third round.
After becoming the mandatory challenger, Brook was set to fight Devon Alexander for the world title on 19 January 2013 at the Nokia Theatre in Los Angeles. However, Brook announced he had sustained an ankle injury and the fight was pushed back until 23 February 2013 and the venue changed to the Mosaic Youth Center on Grand in Detroit. Then Alexander announced he had a bicep injury causing the fight to be rescheduled yet again. However, Kell would later pull out of this fight again after re-injuring his ankle, with fellow Brit and Matchroom Stable mate Lee Purdy taking his place but retiring in the 7th round.
After the saga with Devon Alexander, Brook took a fight with Carson Jones in Hull on 13 July 2013, in a rematch of their original fight just over a year previously, which Brook won via a narrow majority decision. Brook took the early rounds of this contest looking far too powerful and quick for the American. Kell went on to win by stoppage in the 8th round to take his record to 30-0.
It was announced shortly after the rematch with Carson Jones that Brook would next face Vyacheslav Senchenko at the Motorpoint Arena in Sheffield on 26 October 2013. Senchenko had recently beaten Ricky Hatton forcing him to retire for good from boxing. Brook said he was using this as motivation and wanted to "get revenge" for a British boxing hero and a personal friend within the sport. Hatton was ringside for the fight and spoke to Kell in the dressing room just prior to the fight. The opening and second rounds were very cagey encounters with neither fighter wanting to give too much away with Brook perhaps using his jab more effectively and taking both the rounds. In the third Brook knocked his opponent down with a powerful right with Senchenko taking the 8 count and continuing. Brook started the fourth strongly out boxing his Ukrainian opponent although Senchenko did unsteady Brook for a few seconds before Brook sent Sencheko back to the canvas with another right. Referee Shada Murdaugh concluded that Senchenko was in no position to continue and Brook was declared the winner by technical knockout.
After beating Senchenko, Brook was once again the mandatory challenger for the IBF welterweight world title. Shawn Porter was the favorite to win and came out aggressively as he pressurized Brook, relying heavily on looping shots. Brook kept his composure, however, and showcased formidable defensive attributes, strength, and clean punching throughout the fight. Brook's technical ability took over after the middle rounds as the straight one-two combinations hurt Porter. Brook defeated Porter to win the title at the StubHub Center in Carson, California on 16 August 2014 by a majority decision (114–114, 116–112, and 117–111). The fight did not generate as many viewers as expected, an average of 661,000 viewers on Showtime. The whole card averaged just 514,000 viewers.
Six months after sustaining a life-threatening injury where he was stabbed three times in the leg and once in the arm during a machete attack, Brook fought his mandatory challenger, Jo Jo Dan. The fight took place on 28 March 2015 in Brook's home town, Sheffield. Brook came out landing straight punches. In round two, a straight right hand wobbled Dan. Brook followed up with a right uppercut that sent Dan to the canvas. Soon after, Brook landed two straight right hands, the first wobbling Dan and the second creating another knockdown. In the fourth round, Brook threw a straight right and left hand that hurt Dan and followed up with a furry that resulted in the third knockdown. The final knockdown was scored at the bell via counter left hand. Dan's corner subsequently stopped the fight in between rounds.
On 30 May 2015 at The O2 Arena, Brook successfully defended his IBF welterweight for the second time just two months after his previous fight. Brook landed his jab from the start and picked up the pace as the fight went on, cutting off the ring and walking through his opponent. Gavin remained defensive but did occasionally find success. In round six, Brook showed versaltility in throwing a variety of combinations, mixing soft and hard punches, while Gavin attempted to avoid trading. As Brook landed an uppercut however, Gavin exchanged a right hand, which was countered by Brook's straight left hand. Gavin staggered back to the ropes, and the referee stopped the fight as Brook threw another flurry.
On 26 March 2016 after nearly a year without fighting, Brook stopped mandatory challenger Kevin Bizier in the second round. En route to landing a 'big fight,' Brook took on his second mandatory challenger since winning the IBF World title in 2014. The fight took place at Sheffield Arena, in South Yorkshire. After the bout, Brook and Eddie Hearn talked about potential unification fights against the likes of Tim Bradley, Danny García, Keith Thurman and Jesse Vargas. Brook rocked Bizier with a left to the jaw and he then further dazed Bizier with a series of hooks. After taking a count, Bizier did not look that interested in carrying on and Brook unleashed a combination that left the Canadian crumpled in a heap.
It was announced on 12 May 2016 that negotiations for a unification fight with WBO titleholder Jessie Vargas (27-1, 10 KOs) were under way. Eddie Hearn told Sky Sports he had "offered a huge amount of money" to make the fight happen and while he conceded Brook might have to travel to America, Vargas insists he is willing to cross the Atlantic. Danny Garcia, the WBC champion, was top of the list, but Hearn conceded the Vargas fight looked more likely. Hearn said the fight will take place 20 August, 27 August, or 3 September at Bramall Lane in Sheffield. Vargas confirmed via his Twitter account that the bout is set for September 3. Negotiations fell through on July 9 for the unification title fight, when it was announced that Brook would be moving up two weight divisions to challenge Gennady Golovkin instead.
On 8 July 2016, it was announced that Brook would face undefeated Unified Middleweight titleholder Gennady Golovkin (35-0, 32 KOs). The fight took place on 10 September 2016 at the O2 Arena in London. Brook was scheduled to fight in a unification bout against Jessie Vargas. Likewise, there were negotiations for Golovkin to fight Chris Eubank Jr. However, negotiations fell through and Brook agreed to move up two weight divisions to challenge Golovkin. The fight aired in the United States on HBO and on Sky Box Office pay-per-view in the United Kingdom.
On September 5, the WBA withdrew its sanction for the fight. Although they granted Golovkin a special permit to take the fight, they stated that their title would not be at stake. The reason for the withdrawal was because Brook had never competed in the middleweight division. WBA president Gilberto Mendoza Jr. said, "What I most regret is that there are no boxers at 160 pounds who will fight against 'Triple G,' and Brook has to move up two divisions to fight against him." The Golovkin camp were said to be disappointed with the decision with promoter Tom Loeffler saying, "somehow the WBA thought it was too dangerous for a welterweight to move up to middleweight to fight the biggest puncher in boxing. I guess that is a compliment to GGG as they sanctioned [Adrien] Broner moving up two divisions [from lightweight to welterweight] to fight Paulie [[[Paulie Malignaggi|Malignaggi]] in 2013] and Roy Jones moving up two divisions [from light heavyweight to heavyweight] to fight John Ruiz [in 2003] for WBA titles, and Kell Brook is undefeated and considered a top pound-for-pound boxer."
On the night, Brook was staggered early in the first round, but kept his composure, growing stronger as the rounds unfolded. In the opening rounds, he was trading blows with Golovkin, but suffered a damaged eye socket, and after heavy punishment in the fifth, trainer Dominic Ingle threw in the towel, losing by TKO. Despite this, Brook won many admirers for his spirited performance stepping up two weight divisions to make the fight. Ingle confirmed Brook had gone through surgery successfully. According to Compubox punch stats, Golovkin landed 133 of his 301 punches thrown (44.2%), whilst Brook landed 85 punches, having thrown 261 (32.6%). The fight was aired live on HBO in the afternoon and drew an average of 843,000 viewers and peaked at 907,000 viewers. This was considered to be a huge success for an afternoon showing. A replay was shown later in the evening as part of the world super flyweight title fight between Roman Gonzalez and Carlos Cuadras. The replay averaged 593,000 viewers.
In January 2017, Brook and his team were in talks with Amir Khan over a potential grudge fight, whilst also keeping the mandatory fight with Spence as second choice. During negotiations, Khan urged Brook to fight Errol Spence first and eventually talks broke down between Brook and Khan. Eddie Hearn mentioned talks were already ongoing with Spence's manager Al Haymon for a fight to take place possibly in the UK in May. Hearn received an extension from the IBF for negotiations between himself and TGB Promotions boss Tom Brown, as they were progressing. The purse bids for the fight was set for February 7 by the IBF. On 1 February, Hearn claimed that Brook will be keeping his title and make the defense against Spence and that he had reached out to Spence's team, to no reply. He assumed that they wanted the fight to go to purse bids. Spence said that he had no problem travelling to the UK for the fight, regardless of negotiations. With a deal close to being reached a day before the purse bids, the IBF granted a week extension, pushing the purse bid back 7 days. On 13 February a deal was reached for the fight to take place in Sheffield on 20 May 2017. At a press conference at Bramall Lane, Sheffield on 22 March, the fight was officially announced to take place on 27 May 2017 live on Sky Box Office in the UK and Showtime in the US.
In front of 27,000 fans in attendance, Brook lost his IBF welterweight title after being stopped in round 11. In a fight where mostly power shots were landed, Spence threw combinations to the head and more to the body, wearing down Brook. Brook did well working the counter, and landed his own shots to the body, but Spence came back with his own shots. In round 10, Spence, having Brook against the ropes unloaded some heavy power shots, which caused Brook to take a knee. In round 11, before the fight was stopped, Brook motioned many times that he couldn't see through his left eye and voluntarily took a knee, again counting as a knockdown. The referee started the 10 count which Brook couldn't make, giving him back to back defeats and his first as a welterweight. His trainer Dominic Ingle was also on the apron before the fight was officially stopped. At the time of stoppage, all three judges had Spence ahead on their scorecards (97-92, 96-93, 95-94). In the post-fight interview, Brook credited Spence admitting he lost to the better boxer, “It was a very tough fight. I would say Spence is one of the best kids I’ve been in with, if not the best. I got caught in the seventh round, in the eye, and it felt like the same as in the Golovkin [fight]. Probably not as bad as that time, but I was getting hit on it over the rounds and it was getting worse. The last round, he caught me in it and I just stood there, double vision, and I couldn't see out of the eye [...] so I had to stop.” Spence landed 246 of 633 punches thrown (39%) whilst Brook landed 136 of 442 (31%). The fight was shown in the afternoon in the US on Showtime and averaged 291,000 viewers, peaking at 337,000 viewers. This was considered low numbers, even for an afternoon showing, possibly due to it being a holiday weekend. Brook spoke to Sky Sports the following day and confirmed that a CT scan showed he had broken his eye socket, meaning he would likely have surgery.
On 30 May, Brook admitted he may have to move up to light middleweight. Brook had successful surgery on his eye in June 2017 and said that he would start training again in the summer looking to make a ring return in late 2017.