John was born to a working class family in Manchester. His mother left when he was seven, and he did not see her again for many years.
He went to the RADA (Royal Academy of Dramatic Art). His biggest role in the 70s was as Regan in The Sweeney, in the 80s as the eponymous Inspector Morse, and the 90s as Kavanagh QC. He was married twice, his first marriage Sally Alexander lasted for four years (1964 - 1968) and his second marriage to Sheila Hancock lasted until he died. From 24 December 1973 until 21 February 2002.
Thaw had two daughters: Abigail Thaw from his first marriage, and Joanna Thaw from his second. He also had a step-daughter, Melanie-Jane, Sheila's Hancock's daughter. Abigail is also an actor.
He died at age 60 from cancer.