Algieri was born into a middle-class family in Huntington, New York, to an Italian father and an Argentine mother. He has a bachelor's degree in healthcare science from Stony Brook University and a master's degree from the New York Institute of Technology. Algieri has expressed his desire to go to medical school and combine his experience as a world-class athlete and his knowledge of nutrition to spearhead a practice designed to treat competitive athletes without the use of unnecessary surgeries, pharmaceuticals or long lay-offs.
Algieri began his martial arts training in Chinese Kenpo under the tutelage of Robert Mauro at the age of 10. Algieri received his black sash at the age of 15 and began competing in amateur kickboxing at the age of 16. While in high school, Algieri started wrestling and eventually became captain of the St. Anthony’s varsity squad. Algieri won two Catholic League titles and one Catholic State Championship, ultimately making it to the semi-finals of the Public State Tournament. He was also awarded a wild card nomination to compete in the High School National Championships, but a knee injury barred Algieri from participating. After high school, Algieri spent three seasons as assistant coach to the St. Anthony’s Friar Wrestling team. Algieri attended Stony Brook University and concentrated his athletic prowess to Full-Contact Kickboxing. He then began sparring and training with Tim "Bring the Pain" Lane under the watchful eye of Mauro. After completing an undefeated amateur career that culminated with a USKBA Northeast Championship, Algieri decided to turn professional at the age of 19.
As an amateur, Algieri won the USKBA Northeast Championship defeating Kyle Murray. Algieri later turned professional, and won the International Sport Karate Association Welterweight and World Kickboxing Association Super Welterweight Championships, compiling a 20-0 record as a world kickboxing champion before retiring to become a professional boxer.
Algieri turned pro in 2008, and put together a record of 18-0 with 8 knockouts. Most fights, he moved between light welterweight and welterweight. On February 14, 2014, Algieri won a unanimous ten round decision over Emmanuel Taylor, an IBF number four ranked contender, at the Paramount Theatre in Huntington, Long Island, New York, to enter the world rankings.
Algieri had accumulated wins over the likes of top prospect Jose Alejo, a bout featured on NBC Sports Network and former United States Boxing Association (USBA) Champion Mike Arnaoutis prior to the Taylor fight, and he reached 10-0 when Joe DeGuardia's Star Boxing signed him.
On June 14, 2014 Algieri got the chance of a lifetime as he faced and defeated the newly crowned WBO junior welterweight champion Ruslan Provodnikov, headlining on HBO's "Boxing After Dark" card at the Barclays Center in Brooklyn, New York. Algieri was seen as a warm up fight for Provodnikov, which seemed to be the case as early on Algieri was dropped and hurt twice, with his eye swelling grotesquely, essentially blinding him for the rest of the fight, it all seemed to be over. However, Algieri fought back and began to box very accordingly for the remainder of the fight, using his strong jab to keep his opponent back and moved on the outside, avoiding his Russian counterparts big looping shots. The bout went the full 12 rounds and Algieri won via split decision to become the new WBO junior welterweight champion.
After proving himself in the Provodnikov fight, Algieri was given a big shot as he fought legendary boxer Manny Pacquiao on November 23, 2014 in Macau, for the latter's WBO Welterweight title, it was Algieri's first fight outside the State of New York. Algieri was stripped of his belt at 140 as WBO rules mandate a fighter of theirs can only hold belts in one division at a time. Algieri was a big underdog coming in, and in a relatively one sided bout, Pacquiao knocked Algieri down six times and made him tumble repeatedly en route to defeating him via one-sided unanimous decision (119–103, 119–103 and 120–102), ending Algieri's undefeated streak.
On April 3, 2015, Amir Khan announced via his wife's YouTube channel that he had agreed to a May 30 welterweight bout with Algieri in New York. The fight was later finalized on April 15, and subsequently set for May 29 at the Barclays Center in Brooklyn, New York. Algieri coming off the Pacquiao loss was seen as an easy touch for Khan by many. But with his new trainer John David Jackson, Algieri hatched a fight plan to be aggressive and apply pressure in the bout, when he usually fights on the outside with a strong jab. He entered the bout weighing more than Khan, and in the fight many commented on how much Algieri had improved since the Pacquiao fight, and that he looked the stronger and fitter of the two. Algieri instantly began cutting down the ring and had Khan backing up fast and unable to land his quick combinations. Algieri, who is not really known as a puncher was able to rock Khan with a big shot early on, and Algieri chased Khan the whole night, landing big punches in every round and finished the night strongly. Khan went on to win the fight via unanimous decision after 12 rounds of boxing, many believing him to win widely on the cards as he was able to steal many of the rounds in the last 10 seconds. Algieri however, believed that he had in fact won the bout.
As of December 12, 2015, Algieri's career record stood at 21-2 as a professional boxer, with 8 wins coming by way of knockout.
On December 5, 2015, Algieri knocked down Ecuadorian Erick Bone (16-2, 8 KOs) in the 8th round to win via unanimous decision. Bone was coming off a 5th round knockout loss to Shawn Porter. The judges, after ten rounds, scored it (95-94, 97-92, 97-92).
On February 23, 2016 it was announced that Algieri would take on one of welterweights hottest prospects Errol Spence Jr. (19-0, 16 KOs) on April 16 at the Barclays Center where he was (2-2) against opponents. This fight was important to both Algieri, who had to redeem himself in the division after losing to Khan and Pacquiao, and important to Spence as he wanted to make a statement in the division as a rising star against a former titleholder. Spence dominated the bout with jabs in center ring and won every round. Algieri moved well and tried to both land jabs and counter. Algieri went down once in the fourth round and twice in the fifth round, the first time was on his back. the second time Spence finished him on the ropes. The time of stoppage was 0:48 of round 5. It was the first time Algieri had been stopped inside the distance. Algieri received a purse of $325,000 with which he was unhappy. Spence earned $225,000.