O'Sullivan grew up and still lives in the exclusive Manor Road area in Chigwell, Essex and attended Wanstead High School. He started his career at an early age. He first achieved a century break at the age of 10, making a 117; scored a 147 or maximum break at 15; and turned professional at 16. He won his first 38 matches as a professional – a record that still stands[ – on his way to the 1993 World Championship, where he remains the youngest-ever player to qualify. He was the youngest ever winner of a ranking tournament when he won the 1993 UK Championship aged 17, beating Stephen Hendry in the final, and starting one of the most prominent rivalries in the sport. In 1995, he won his first Masters title.

On 21 April 1997, in the first round of the World Championship against Mick Price, he made the fastest-recorded maximum break (and also his first), 5 minutes and 20 seconds, an average of one shot every 9 seconds. In November, he won his second UK title, beating Hendry 10-6 in the final.

O'Sullivan made it to the semi-finals of the 1999 Embassy World Championship, losing 17-14 to Stephen Hendry, in a match featuring many century breaks between the two players.

In 2001, O'Sullivan claimed his first World Championship title (dedicated to his father), with an 18-14 victory over John Higgins, and his third UK title, with a 10-1 victory over Doherty. He began the 2002/2003 season ranked number 1.

In 2003, following a season that saw him win the Scottish Masters, the European Open and Irish Masters, O'Sullivan's season ended on another disappointing note when he was knocked out of the World Championship in the first round for the third time in his career, losing 10-6 to the unseeded Marco Fu, despite making another maximum 147 break in the match. This defeat saw him drop to number three in the rankings.

In the 2005 World Championship, he lost to Peter Ebdon in the quarter-final. From 8-2 down, Ebdon began a comeback and eventually won 13-11, by playing in an exceptionally determined and dogged style, with many observers accusing him of deliberate slow play to disrupt O'Sullivan's fast game. After the match, O'Sullivan indicated to the press that he was unlikely to compete in the following season, and perhaps even would retire from the sport altogether.

The 2006 World Championship saw O'Sullivan's personal sponsor, 888.com, also become the event sponsor for the following 2 years. On his way to losing the 2006 Northern Ireland Trophy, where he lost 9-6 to Ding Junhui in the final, he defeated semi-final opponent Dominic Dale 6-0 in a record 53 minutes for a best-of-11 frame match.

During the 2007 Northern Ireland Trophy O'Sullivan set a new record after compiling five centuries in a 5-2 defeat of Ali Carter. This also included his seventh, official competitive 147 maximum break. O'Sullivan went out of the tournament in the next round after being beaten by Fergal O'Brien. On December 2, 2007, he won a fourth consecutive, and record seventh in total, Premier League title by beating John Higgins in the final by a score of 7-4.

On December 15, 2007, O'Sullivan compiled his eighth maximum break in competition in the deciding frame of his Maplin UK Championship semi-final against Mark Selby at Telford, equaling Hendry's record. In doing so, O'Sullivan also became only the second person in professional competition to compile a maximum in the deciding frame of a match. Hendry had made the first against O'Sullivan in the 1997 Liverpool Victoria Charity Challenge final. O'Sullivan is also the second player (after John Higgins) to make 147 breaks in two consecutive ranking tournaments (Northern Ireland Trophy 2007 and Maplin UK Championship 2007). He then went on to win the tournament easily beating Stephen Maguire 10-2 in the final (from 8-0 up), and picked up a £100,000 cheque for winning his first ranking tournament in almost three years.

He compiled a record breaking ninth competitive 147 break at the Crucible against Mark Williams at the 2008 World Championships. It was his third of the season, and also his third maximum at the Crucible. No other player has made more than one maximum at The Crucible. It was the fourth maximum to be compiled in a winning frame of a match.

O'Sullivan plays in a fast and attacking manner. He is a prolific breakbuilder and solid tactical player, although he has stated his disdain for long, drawn-out games, saying that it harms the game of snooker. He is a good front-runner, although tends to become demoralized when behind and not playing well, and is liable to lose multiple consecutive frames. O'Sullivan is right-handed, but can play to a high standard with his left hand.

He is considered by many to be the most naturally-talented player in the history of the sport, with many such as Steve Davis labeling him a 'genius'; some have said that his highest level of play is above any other player's highest level of play; and many have hailed him as the greatest player ever.