Mark Cavendish began his career with the British Track Cycling team. He won gold in the madison at the Los Angeles World Track Championships with Rob Hayles. They had not raced together before. They finished one lap ahead of the field to claim the gold medal, ahead of the Dutch and Belgian teams, giving Britain its fourth gold at the championships. It was Cavendish's first world champion's jersey. Cavendish also won the 2005 European championship points race. He began road racing in 2005, riding the Tour of Berlin and Tour of Britain as a trialist with Team Sparkasse.
Cavendish began 2006 with the Continental team, Team Sparkasse, a feeder squad for T-Mobile Team. In June, he won two stages and the points and sprint competitions in the Tour of Berlin. He rode for the Isle of Man on the track at the 2006 Commonwealth Games in Melbourne, riding the scratch race. He lapped the field with three others: Rob Hayles; Ashley Hutchinson of Australia; and James McCallum of Scotland. He then beat these in the sprint to win gold for the Isle of Man. The race time was 23m 5s, an average 51.9 kilometres per hour (32.2 mph).
His success at the Tour of Berlin led to a post as a "stagiare" with T-Mobile from August until the end of the season. His best result for T-Mobile in 2006 was in the Tour of Britain, where he came second three times and won the points classification. It brought a full professional contract for 2007 and 2008.
Cavendish began 2007 "laboriously", said the daily sports paper, "L'Équipe". It quoted one of Cavendish's team-mates, Roger Hammond: "To be honest, he started the season so catastrophically that the staff were wondering what they could enter Mark for so that he could finish the race.". Cavendish's breakthrough came at the Grote Scheldeprijs race in Belgium, which he won overall after failing to even finish any of his previous races. He went on to win stages at the Four Days of Dunkirk and the Tour of Catalonia Volta a Catalunya, the third oldest cycling stage race in the world, and that brought selection for the 2007 Tour de France, starting in London. He crashed in stages 1 and 2. and abandoned on stage 8 as the race reached the Alps, having taken two top-ten placings but unhappy not to have had a top-five placing.
Then the pattern changed. "Everything fell into place," "L'Équipe" said. His debut season continued, moving on to 10 stage wins, one behind Alessandro Petacchi's record 11 for a début season. Cavendish took his 11th win in early October, the Circuit Franco-Belge, to equal Petacchi's record. Among the wins were three in UCI ProTour events, the two in the Volta a Catalunya and one in the Eneco Tour of Benelux.
In 2008, Cavendish returned to the track, winning the Madison World Championships in Manchester with Bradley Wiggins, as Great Britain topped the medal table.
On the road, Cavendish won his first stages of a "grand tour", by picking two victories in the 2008 Giro d'Italia. Cavendish won four further stages in the 2008 Tour de France, his first coming in Stage 5 from Cholet to Châteauroux. He won again on Stage 8, Stage 12 and Stage 13, making him the first British rider to collect four stages in a single Tour. Overnight, at the age of just 22, he became the "fourth most successful British professional in history", said "The Independent". After Stage 14, Cavendish abandoned the Tour to concentrate on the Beijing Olympics. He and Team Columbia manager Bob Stapleton agreed that riding the Alps was a risk to his hopes. But Cavendish, with Bradley Wiggins, failed to win a medal, finishing joint eighth in the madison. He was the only British track cyclist not to win a medal. Following the Olympics, Cavendish remained angry with British Cycling for not giving much attention to the madison, though Chris Boardman said that Cavendish's professional commitments also interfered with his build up to the Olympics. In November 2008, Cavendish said that he had no further plans to return to track cycling.
The rest of his season was also successful, with a total of eleven further race wins, including three each at the Tour of Ireland which he abandoned on the final day, and the Tour of Missouri, winning his only points classification of the season at the latter. At the Tour de Romandie, Cavendish won the prologue time-trial, beating compatriot Bradley Wiggins and emphasising his short-distance time-trial abilities.
Cavendish's 2009 season began at the Tour of Qatar, where he renewed his rivalry with Tom Boonen. Boonen won the race and one stage, though Cavendish took two stages; he also won two stages at the Tour of California, again beating Boonen in the sprint finishes.
The Tour of California also saw Cavendish win his first points of classification of the 2009 season.
Cavendish was a surprise inclusion in the British squad for the 2009 UCI Track Cycling World Championships, where he competed in the Scratch Race and the Madison, failing to pick-up medals in either.
He took up the European season at Tirreno–Adriatico, the Italian one-week stage race, where he won one stage.
He then entered his first classic race, Milan – San Remo, and, after a week of uncharacteristically humble pre-race statements, rode effectively over the climbs that his rivals had said made this race impossible for him to win - and then tracked down Heinrich Haussler in the last 200 meters to narrowly win the sprint and the race, Cavendish's first victory in a race known as one of the "five monuments of cycling".
Cavendish repeated his two-stage victory at the Three Days of De Panne from 2008, also winning the points classification.
At the start of the 2009 Giro d'Italia Columbia won the Team Time Trial and he was given the Maglia Rosa leaders jersey, becoming the first British rider to ever wear it. The first 2 road stages however were fruitless for Cavendish, who was beaten to the line by Alessandro Petacchi in the first stage and was caught behind a crash and failed to make it back for the sprint the next day. Cavendish soon asserted his sprinting dominance on the race however, gaining 3 stage wins before abandoning following stage 13, citing a need to rest for beginning preparations for the Tour de France. He continued his preparation by racing the Tour de Suisse. He won both stage 3 and stage 6 to wear the points jersey. However, this was taken off him by Fabian Cancellara.
In June he published his first autobiography, "Boy Racer", which covered his career to date. At a press conference in London ahead of the 2009 Tour de France, Cavendish explained that the book was too premature to be seen as a complete biography and that his "biggest motivation for writing it had been to explain himself better" due to the way he perceives himself when performing post-stage interviews. Instead, he felt that the book was "more a biography of last year's Tour stage wins".