In his early 20s, Horsfall began competing in British club level events and he won The Motor Trophy at the MCC's Llandudno Trials in 1934, driving a Wolseley.
[2] During the late 1930s, he became a familiar and popular competitor at British and European motor races, usually driving his Aston Martin 2-litre Speed Model, nicknamed the 'Black Car', or Tony Rolt's ERA.
[4] In the same year the pairing took victory in the 2-litre class and finished second overall in the RAC Tourist Trophy race at Donington Park, beating the more fancied BMW works cars.
As part of his duties he was involved in Operation Mincemeat, a famously successful disinformation plot to convince the Germans that the Allies planned to land in Greece rather than Italy.
[8] Having finished in sixth place in the first 20-lap heat, on lap 13 of the 30-lap final race he clipped a straw bale at Stowe Corner and the car rolled.