Collins joined Cardiff City as a trainee in August 2000, after playing local football in his native city of Newport. He made his debut for Cardiff in an FA Cup match against Bristol Rovers in November 2000, which Cardiff won 5–1, and went on to make 86 appearances in all competitions for Cardiff, scoring six goals. He won the Welsh Young Player of the Year award in 2005.
Collins joined West Ham United in July 2005, along with his centre-back partner at Cardiff, Danny Gabbidon, in a £3.5 million deal for the pair as Cardiff attempted cut their wage bill and raise money.
Due to fitness problems as well as competition from more experienced players, Collins did not make his West Ham debut until the League Cup tie against Sheffield Wednesday on 20 September 2005, which West Ham won 4–2. He made his Premiership debut on 29 October 2005 in a 2–0 away defeat by Liverpool. He scored his first goal for West Ham in a 1–1 draw with Portsmouth on Boxing Day 2005. Collins ended his first Hammers season with two league goals, after scoring again in a 4–1 defeat against Chelsea at Stamford Bridge in April 2006.
Due to injuries to new signing Matthew Upson and 2005–06 Hammer of the Year Danny Gabbidon, Collins was given the chance to form a centre-back partnership with Anton Ferdinand towards the end of the 2006–07 season.
On 28 January 2008, it was announced that Collins would be out for a year after suffering severe ligament damage in a West Ham reserve game against Portsmouth. His injury kept him out for nine months and on 26 October 2008, Collins returned to first team action, starting in West Ham's 2–0 loss to Arsenal at Upton Park.
Collins signed a four-year contract with Aston Villa for an undisclosed fee on 1 September 2009, the last day of the transfer window. On his league debut for Villa, on 13 September, he helped them keep a clean sheet in the Second City Derby against Birmingham City. His first goal for the club was the winner against Chelsea in a 2–1 victory, in which he was named "Man of the Match". In March 2011, manager Gérard Houllier fined Collins and fellow centre-back Richard Dunne two weeks' wages after what was reported as "a boozy row with club staff during a team-bonding exercise".
On 1 August 2012, Collins signed for West Ham United for an undisclosed fee on a four-year contract. He was allocated the number 19 shirt, which he wore in his first spell with the club. On 18 August 2012, Collins started alongside Winston Reid in a 1–0 home win against his former club, Aston Villa on the opening day of the 2012–13 Premier League season. He was also awarded 'Man of the Match' for his performance on his second 'debut'. Collins first goals in his second spell with West Ham came on 5 January in an FA Cup 3rd round tie against Manchester United. The game finished 2–2 with Collins scoring two headed goals from crosses by Joe Cole. Collins signed a two-year extension to his contract on 24 December 2015, which could keep him at the club until 2018. On 15 August 2016, Collins scored West Ham's first Premier League goal of the season, in a 2–1 defeat against London rivals, Chelsea.
Collins played for the Wales under-21 team on several occasions and captained them in a match against Germany U21's.
Collins made his debut for the senior Wales side against Norway in 2004 and scored his first goal for Wales in a 3–1 defeat to Cyprus on 13 October 2007. Ever present during 2010 World Cup qualifying, he captained Wales in the last qualifier against Liechtenstein and helped keep a clean sheet in a 2–0 win. Collins was given the captain's armband for the senior squad for the first time for a friendly against Sweden on 3 March 2010. Collins scored his first international goal in the 21st minute of a Euro 2008 qualifying match against Cyprus in October 2007. On 7 September 2012 in a 2014 World cup qualifying against Belgium game he was given a straight red card for a two-footed lunge on Guillaume Gillet, Wales went on to lose the match 2–0.
Collins is redhead and is nicknamed "Ginge" and "Ginger Pelé".
Correct as of match played 16 September 2017As of match played 5 September 2016
Cardiff CityFAW Premier Cup: 2001–02
Football League One play-off winners: 2002–03
West Ham UnitedFA Cup: Runner-Up 2006
Aston VillaLeague Cup: Runner-up 2010