Onley was born in London and grew up in Kelso, in the Scottish Borders. He joined the local club, the Kelso Wheelers, at ten, though cross-country running remained his main sport through childhood. He committed to cycling at eighteen, having found he preferred the training for it. Two years with the Van Rysel-AG2R La Mondiale development team followed, the second of them broken up by the pandemic.
A move to Development Team DSM came in 2021, with backing from the Rayner Foundation. At the 2022 CRO Race he went with Jonas Vingegaard on several stages, took third overall and won the young rider classification. DSM signed him to a five-year contract late that year and promoted him to the WorldTour squad.
He was in the DSM team that won the opening team time trial of the 2023 Vuelta a España, his first Grand Tour, then crashed on wet roads the next afternoon and broke his collarbone. The following January brought a first professional win, on the uphill finish of stage five at the Tour Down Under, where he also took fourth overall.
At the 2024 Tour de France he made the break on consecutive Alpine days, taking fifth on stage seventeen and reaching the foot of the Col de la Bonette in the leading group the next morning, before finishing 39th overall. The Tour of Britain that September opened with a stage from Kelso to Kelso; he led the youth classification from stage two and was second overall, sixteen seconds behind Stephen Williams.
He was 16th at the 2024 world championships in Zurich, the best British finisher. Fourth again at the 2025 Tour Down Under, he won stage five of the Tour de Suisse and finished third overall behind João Almeida and Kévin Vauquelin. At the Tour de France he was fourth in Paris, twelve minutes and twelve seconds behind Tadej Pogačar and second to Florian Lipowitz in the young rider classification.









