English character actor Jonathan Newth was trained at the Central School of Speech and Drama in London, alongside contemporaries Julie Christie and James Bolam. On stage from 1957, he first acted in repertory theatre and in London's West End before making his Broadway debut in The School for Scandal in 1963. Newth spent several seasons with the Royal Shakespeare Company in Stratford-upon-Avon, where his roles included the Player King in Hamlet, Leonato in Much Ado About Nothing and Duke Frederick in As You Like It. More recently (2008-2009), he played Don Alejandro de la Vega, father of the heroic protagonist, in Zorro at the Garrick Theatre in London.

On screen from 1964, Newth has been seen in diverse roles, ranging from clerics (Coronation Street (1960), Emmerdale Farm (1972)) and military men (Wings (1977), Tenko (1981), The Return of Sherlock Holmes (1986)) to magistrates (Jacobean barrister and judge Sir Edward Coke in The Gunpowder Plot (1968)) and noblemen in period drama or literary adaptations (the Vicomte de Bragelonne in The Man in the Iron Mask (1968); Lord Hastings in The Elusive Pimpernel (1969); Anne Boleyn's brother, Viscount Rochford, in The Six Wives of Henry VIII (1970) and Tsar Alexander I in Napoleon and Love (1974)). In addition to numerous guest spots, he has enjoyed recurring appearances in the medical soap Emergency-Ward 10 (1957), the family drama The Brothers (1972) (art tutor Nicholas Fox), the sitcom After Henry (1988) (amiable bookshop owner Russell Bryant) and the original series of Poldark (1975) (as Verity's husband, Captain Andrew Blamey).

Newth has been married to the actress Gay Wilde since 1979.