He rode for Landbouwkrediet–Colnago in 2005 and 2006, recording second-place finishes at the 2005 British National Road Race Championships and the 2006 Trofeo Laigueglia, to Russell Downing and Alessandro Ballan respectively.

[8][9] In 2007 he switched to Discovery Channel, making his first start at a Grand Tour by riding the Giro d'Italia – as a late replacement for defending race winner Ivan Basso.

[12] In his second start with the team, Cummings won stage 2 of February's Giro della Provincia di Reggio Calabria.

[13] He again rode the Giro d'Italia, recording a fourth-place stage result on the final summit finish of the race at Monte Pora, having been a part of the breakaway.

He held on to his lead and won by four seconds over the two chasers, Cameron Meyer of Orica–GreenEDGE and Team Sky's Juan Antonio Flecha.

[33] He completed his season with a stage victory on the final day of the Tour of Beijing,[34] getting the better of Ryder Hesjedal in a sprint à deux in Pinggu.

[43] It was the first Tour de France stage win for both Cummings and his South African team MTN–Qhubeka, with the result coming on Mandela Day.

[45] The following month, he won the third stage of the Tour of the Basque Country, outfoxing the peloton with a move with 1 kilometre (0.62 miles) to go, and held on to win by several metres in Lesaka.

[47] After six stages of July's Tour de France, Cummings had lost almost 45 minutes to the race leader Greg Van Avermaet, and was in 191st place out of the 198 competitors.

[49][50] In the same month, he was named to Great Britain's cycling team for the Olympic Games in Rio – replacing Peter Kennaugh for the road race.

[59] Writing for The Guardian, journalist William Fotheringham considered this win for Cummings as the most important stage race victory of his career up to that date.

[60] During the 2017 Tour of the Basque Country, Cummings crashed heavily and required surgery for fractures to his left clavicle and scapula, as well as his sternum.

[7] Cummings endured a difficult 2018 season, recording no top-10 finishes, and suffering health issues with an injury to the head of his left fibula at the Tour of Austria,[64] and breathing problems at the Critérium du Dauphiné.