Educated at University College London, Williamson worked as a reporter on Tribune (1982–84) and was then briefly its literary editor (1984) before becoming editor (1984–87) as successor to Chris Mullin. Just before the 1987 general election he was hired as the editor of the Labour Party's members' magazine Labour Party News (1987–89), to which he added the editorship of the party's monthly New Socialist (1987–89) after Neil Kinnock fired Stuart Weir as editor for advocating tactical voting during the election campaign.
In 1989 Williamson joined The Times as a reporter, becoming diary editor (1990–92), then home news editor (1992–95) and Whitehall correspondent (1995–96). He currently writes on pop and world music for a variety of publications, including the magazines Uncut and Songlines. He has also written books, including The Rough Guide to Bob Dylan and The Rough Guide to the Blues.