Rees grew up in Swansea and joined Plaid Cymru at an early age.
In 1951, he refused to do National Service, on the grounds that he was Welsh, and was sentenced to a year in prison.
[3] In 1964, Rees was elected as Vice President of Plaid,[3] unexpectedly beating Elystan Morgan, who was seen as the more conservative candidate.
[3] Although Rees grew up in an English-speaking family, he learned Welsh as an adult, and it became his main language.
[7] In 1973, Rees adapted the Ulpan system of language learning for Welsh learners, renaming it Wlpan,[8] and from 1975, he focused on running the Centre for Teaching the Welsh Language to Adults, based at what became Cardiff University.