Hooson was educated at Denbigh Grammar School and the University College of Wales, Aberystwyth. He served in the Royal Navy during World War II. He became a barrister, called to the bar by Gray's Inn in 1949, and is a Queen's Counsel, appointed 1960. He was chairman of the Flint Quarter Sessions from 1960 and Merioneth Quarter Sessions from 1962, and a member of the Bar Council from 1965. He has also been a farmer.
Hooson became chairman of the Liberal Party of Wales in 1955 and was elected to the Liberal Party executive in 1965.He contested Conway at the 1950 general election and again at the United Kingdom general election, 1951. He became MP for Montgomeryshire at a 1962 by-election following the death of Clement Davies, as a member of the Liberal Party. He contested the Liberal Party leadership election of 1967, but withdrew in favour of Jeremy Thorpe.
In 1979, he lost his seat to the Conservatives apparently because the local party decided to arrange a festival rather than campaign for him but was then appointed a Life peer as Baron Hooson, of Montgomery in the County of Powys and of Colomendy in the County of Clwyd. He now sits for the Liberal Democrats in the House of Lords. Between 1987 and 1993, Hooson was the President of the International Eisteddfod, held annually at Llangollen.