As a jockey his most notable victory came as an amateur rider on Reynoldstown in the 1936 Grand National. Forced by injury to retire from riding he became one of the most successful National Hunt trainers of all time. He trained the winners of four Cheltenham Gold Cups, two Champion Hurdles. five King George VI Chases, seven Whitbread Gold Cups, seven Hennessy Gold Cups and a Grand National, and was British jump racing Champion Trainer five times. He trained 40 winners at the Cheltenham Festival between 1946 and 1986, a record which still stands as of 2009, and he is commemorated in the title of the Fulke Walwyn Kim Muir Challenge Cup at the Festival. Amongst his notable owners were the Queen Mother and Dorothy Paget. As well as his many National Hunt victories, he trained Dorothy Paget's Aldborough to win the Doncaster Cup and Queen Alexandra Stakes on the Flat.
Walwyn became a household name when, in 1964, his Cheltenham Gold Cup winner Mill House defended his crown against the legendary Arkle in an epic struggle that saw the Irish star win the first of his three Gold Cups.
His cousin, Peter Walwyn, was also a racehorse trainer.