In 2020,Callum moved from Lambourn to Newmarket, having been retained by owners the Gredley Family. That season he rode 77 winners.He had his first opportunity to ride in a Group 1 race and in the 2021 Oaks on Save A Forest, a 40/1 outsider owned by the Gredley family and trained by Roger Varian .The pair came fourth, and then went on to win the listed Chalice Stakes at Newmarket the following month. During the winter of 2023/24 Shepherd rode in Bahrain, where his successes included winning the Bahrain Triple Crown on Isle of Jura for trainer George Scott.In May 2024, he achieved his first victory in a Group race when Seven Questions, trained by Scott, won the Group 3 Palace House Stakes at Newmarket. The same month, he won the Lingfield Derby Trial on Ambiente Friendly for trainer James Fanshawe. Callum secured his first Royal Ascot success in 2024, when he partnered Isle of Jura to victory in the Group 2 Hardwicke Stakes
