Born in Nairobi, Kenya, he was educated in Britain before starting his stage career, firstly spending five years in regional theatre.
He made his London theatre debut in 'The Guinea Pig' at the Criterion Theatre just after the second world war, before parts in plays such as the farce 'Ring Round the Moon’, 'The Mortimer Touch', and 'A Woman of No Importance' by Oscar Wilde at the Savoy Theatre in 1953.
Mervyn's later stage roles included those of O'Trigger in 'The Rivals', Lord Greenham in the play 'Aren't We All? 'and Sir Patrick Cullen in 'The Doctor's Dilemma'. Even though he was well respected on the stage, it was with small screen that he became really identified. His first main small screen role was Sir Hector in the 1962 series 'Saki'. 4 years later, he played the Bishop of St. Ogg's in the BBC comedy series ‘All Gas and Gaiter’s'. At that time, breaking with convention, allowing a giggle at the expense of established religion.
Probably most famous for the portrayal of police chief inspector Charles Rose in the ITV series 'The Odd Man' and its spin-offs 'It's Dark Outside' and 'Mr Rose', which has now been released by Network DVD.
He also played the Hon. Mr. Justice Campbell in the series Crown Court made by Granada TV.
Having taken the part of a Chief Inspector in the 1949 Ealing Studios film The Blue Lamp, in which PC George Dixon first appeared, he then re-emerged in a 1960 BBC 'Dixon of Dock Green' episode "The Hot Seat". He was in the Doctor Who story 'The War Machines' in 1966 and several Carry On films of the late 1960s, and also appeared as Mr. Whitty in the ITC 'Randall and Hopkirk (Deceased)' episode "A Disturbing Case" in 1969.
Usually somewhat type cast as a well-to-do upper class chap, he also appeared in 'The Railway Children' in 1970 and 'The Ruling Class' in 1972.
He was also much in demand for advertising voice overs during the 1970's.
He was married to Anne Margaret Payne Cooke and they had 2 sons, including Michael Pickwoad who in 2010 became the production designer on the revamped BBC Doctor Who series.