Williams was born in Auckland, New Zealand in 1950. His father was Samoan, and his mother a Rarotongan of Samoan descent. He was educated at Mt Albert Grammar School, where he started his rugby career. He became an All Black in 1970 as a wing and distinguished himself in the 1970 South African Rugby Tour where he was a sensation, scoring 14 tries in his 13 appearances and in the international series he scored in each of the first and fourth Tests. This was during apartheid, so with his parentage he was only able to tour after honorary white status was granted.
Williams' international rugby career lasted from 1970 to 1978 in which he played 113 matches (including 38 international Tests) and scored 66 tries in all matches as an All Black (ten tries in Tests), which was a record until beaten by John Kirwan.
After he retired from rugby, he coached a number of club sides in New Zealand. During the 1990s onwards, he has been the national rugby coach for Samoa. He is married and has two sons Gavin and Paul, who also play rugby union: Gavin plays internationally for Samoa and plays club for French side US Dax; and his other son Paul played for the Auckland Super Rugby side the Blues before playing for Sale in the English Premiership and debuting for Samoa in 2010. Williams now coaches at the Ponsonby Rugby Club and the Mt Albert Grammar School Rugby Academy.
Williams was appointed President of the NZRU in 2011.