- Malcolm McDowell as Alex DeLarge
- Patrick Magee as Frank Alexander
- Michael Bates as Chief Guard Barnes
- Warren Clarke as Dim
- John Clive as stage actor
- Adrienne Corri as Mary Alexander
- Carl Duering as Dr Brodsky
- Paul Farrell as tramp
- Clive Francis as Joe the Lodger
- Michael Gover as prison governor
- Miriam Karlin as "Catlady" Weathers
- James Marcus as Georgie
- Aubrey Morris as P. R. Deltoid
- Godfrey Quigley as prison chaplain
- Sheila Raynor as mum
- Madge Ryan as Dr Branom
- John Savident as conspirator Dolin
- Anthony Sharp as Frederick, Minister of the Interior
- Philip Stone as dad
- Pauline Taylor as Dr Taylor
- Margaret Tyzack as conspirator Rubinstein
- Michael Tarn as Pete
The film provided early roles for Steven Berkoff, David Prowse, and Carol Drinkwater, who appeared as a police officer, Mr Alexander's attendant Julian, and a nurse, respectively.[6]