His early successes were in track cycling, in which he was a specialist in the team pursuit. He won three World Championships and was Olympic gold medallist twice, in 2008 and 2012. Thomas had an early win on the road at the 2004 Paris–Roubaix Juniors and later had a senior victory at the 2010 British National Road Race Championships. Leaving track cycling to focus solely on the road, he subsequently found success in both one-day/classic races such as the 2014 Commonwealth Games road race and the 2015 E3 Harelbeke, and in one week stage races, most notably at the 2016 Paris–Nice, the 2017 Tour of the Alps and the 2018 Critérium du Dauphiné.