He is a former Minister of Research, Science and Technology, Communications, Statistics, Information Technology, Transport, Local Government and Broadcasting.
He was a strong supporter of prostitution reform and the legalisation of gay marriage.
His "big gay rainbow" speech during the same-sex marriage debate earned him international recognition and an invite to appear on American television show Ellen, which he eventually refused.
He briefly considered standing for the Auckland mayoralty in 2013.
A fellow of the New Zealand Computer Society, he once bragged to reporters that he owned three iPads.
Williamson resigned as a minister in May 2014 following revelations he contacted a top-ranking police officer after businessman Donghua Liu was arrested on domestic violence charges.
Last year, Williamson had to assure Prime Minister John Key that he hadn't asked anyone to make an approach on his behalf to join the Act party, after suggestions made by Act president John Thompson.
He was also suspended from the caucus after the 2002 election for being openly disloyal to then leader Bill English but was brought back into the fold when Brash became leader in 2003.
He later dented his hopes of a Cabinet position after insisting as transport spokesman that National would introduce road tolls if elected in 2008.
In 2010, he was criticised for making racially insensitive jokes about Pacific Islanders and Muslims. And in September last year he was accused of making sexist and misogynist comments while acting as MC at conference dinner, dressed as TV character the Greatest American Hero.
