Lyon was educated at West Leeds High School and University College, London. He became a barrister, called to the Bar at Inner Temple in 1954. He was a member of the Bar Council and of the Fabian Society. He was also a local Methodist preacher and secretary of Leeds North West Constituency Labour Party.
Lyon was elected Member of Parliament for the City of York in 1966, having first fought for the seat in 1964. He was Minister of State at the Home Office but, as a radical socialist, was sacked. He was defeated in the 1983 General Election by the conservative candidate Conal Gregory.
Whilst at the Home Office, Lyon met and fell in love with one of his civil servants, Clare Short, later a Labour MP and cabinet minister. They married in 1981. He died in 1993 from Alzheimer's disease.
I've Been a Very Silly Man with Clare Short.It's Cost Me My Wife; Labour Minister Clare Wrecks Second Marriage
Jul 20, 1997; Cabinet Minister Clare Short was last night accused of being a marriage wrecker after having a passionate affair with a Labour MP...