During his national service in the army Shannon fought numerous bouts, including against such future British boxing luminaries as Joe Erskine and Henry Cooper.
[2] Boxing out of the Fitzroy Lodge Club, Lambeth, he won the south-east London divisional heavyweight final in 1955 and reached the semi-finals of the all-London championship that same year.
Performance gained mixed reviews on initial release, but even those who disliked the film praised Shannon's work in it and he was said to have brought a new level of authenticity to the portrayal of the London gangster.
Among Shannon's other notable film roles were as Jack in That'll Be the Day (1973), the Agent in Slade In Flame (1975), the caged music executive in The Great Rock 'n' Roll Swindle (1980), Peter Rachman in Scandal (1989), and as a landlord in Stoned (2005).
Other television appearances included: EastEnders, Z-Cars, Fawlty Towers, The Sweeney, The Bill and as London Gangster Charlie Turkel in The Professionals episode Old Dog with New Tricks.