He was educated at The John Lyon School in Harrow, London from 1934 to 1939, and attended one year St Andrews University in 1941 before joining the Royal Air Force Volunteer Reserve.
He was called to the Bar at Middle Temple in January 1951, and practised as a barrister in England from 1951 to 1965, including eight years as Parliamentary Counsel from 1953 to 1965, when he drafted constitutions for Pakistan and for Ghana following independence from the UK.
He left his practice at the bar from 1965 to 1973, spending three years as the Chief Executive of the Royal Institution of Chartered Surveyors; then, after being a co-founder of the Professional Association of Teachers in 1968, he was its first chairman from 1968 to 1972.
In 1972, Bennion brought a private prosecution against the young Peter Hain for criminal conspiracy, in relation to Hain's activities as chairman of the Stop the Seventy Tour Campaign which took direct action to disrupt sporting events involving participants from South Africa in 1969 and 1970, as a protest against the apartheid regime.
During the ten-day trial at the Old Bailey Hain dismissed his defence team, which included barrister Geoffrey Robertson, before being convicted and fined £200.