Thomas has enjoyed success both on the track and on the road. On the track he was a specialist in the team pursuit. He won three world championships and was Olympic gold medalist twice, in 2008 and 2012. Thomas had an early success on the road at the 2004 Paris–Roubaix Juniors and later had senior victories at the 2010 British National Road Race Championships, the 2014 Commonwealth Games road race and his first classic, the 2015 E3 Harelbeke. He has also had a number of stage race overall victories; including the 2011 and 2014 Bayern-Rundfahrt, the 2016 Paris–Nice, the 2017 Tour of the Alps and the 2018 Critérium du Dauphiné.
Thomas won the first stage of 2017 Tour de France, an individual time trial, to become the first Welshman, and only the eighth cyclist from the United Kingdom, to wear the yellow jersey at the Tour de France. On stage 9, when placed second overall, he crashed on a descent and broke his collarbone, forcing him to withdraw from the race.
In the 2018 Tour de France, Thomas took the yellow jersey after winning stage 11, extended his lead after winning stage 12, and retained the lead for the remainder of the event, becoming the third British (and the first British born) cyclist, after Bradley Wiggins and Chris Froome, and the first Welshman, to win the Tour de France.