He enjoyed success as a classical stage actor, then played roles on British television. Shakespearean roles on TV include Demetrius in A Midsummer Night's Dream (1968), Gratiano in The Merchant of Venice (1973) and Edmund in King Lear (1975). An early recurring television role was as civil servant Dowling in the final series of boardroom drama The Power Game in 1969.
In 1970 he played Henry Ireton in Cromwell. In 1971, he starred as Tsar Nicholas II of Russia in the film Nicholas and Alexandra, then in 1973 took the lead role of Mr Rochester in a BBC adaptation of Jane Eyre opposite Sorcha Cusack. He appeared as Gratiano opposite Laurence Olivier as Shylock in the National Theatre's film The Merchant of Venice (1974). He made two appearances in the anthology series Thriller in 1974 and in 1975 played Quiller, a spy who never used a gun, in the British TV series of the same name. He appeared as Dornford Yates' gentleman hero Jonathan Mansel in the 1977 BBC adaptation of She Fell Among Thieves. In 1979 he played Peter Guillam opposite Alec Guinness in the serial Tinker, Tailor, Soldier, Spy. He also played Mr Spooner in series Tracy Beaker Returns in 2010.
In 1986 Jayston played the role of the Valeyard in the long-running British science fiction television series Doctor Who. In the serial The Ultimate Foe, the Valeyard is revealed to be an evil version of the Doctor himself. He later reprised the part of the Valeyard in He Jests at Scars..., an audio play in the Big Finish Productions' Doctor Who Unbound series.
Jayston played Neville Badger in the 1989 television adaptation of David Nobbs's comedy of manners A Bit of a Do. He portrayed James Bond in a radio adaptation of You Only Live Twice in 1990. In 1991 he appeared as Colonel Mustard in the television series Cluedo and a year later made a guest appearance in the Press Gang episode "UnXpected". Other TV appearances include in EastEnders, Coronation Street, Only Fools and Horses, The Darling Buds of May, Tales of the Unexpected, The Bill and the character of Donald De Souza in Emmerdale. He also was on Holby City and Tracy Beaker Returns