In the late 1950s and early 1960s Sonny Cullen was one of the best amateur cyclists in Ireland, winning three back-to-back editions of the Jack Woodcock Memorial race (1955–57), as well as two consecutive editions of the Dun Laoghaire Three Day race in 1960 and 1961. In 1960 Cullen was selected to compete in the individual road race at the Roma Olympics but did not finish the course. After the Games he had further success back home, including winning stages of the Rás Tailteann race in 1962 and 1963. Cullen continued to ride for several years and won another Jack Woodcock Memorial race in 1971 before competing in veteran races in the early 1980s.