Born in 1968 to Welsh speaking parents, Siôn Simon grew up in North Birmingham and was educated at Handsworth Grammar school and Magdalen College, Oxford.

After graduating in Philosophy, Politics and Economics, Siôn became research assistant to George Robertson MP, now Lord Robertson of Port Ellen. This was followed in 1993 with a spell as a Senior Manager at Guinness PLC. Between 1997 and 2001, Siôn wrote weekly columns in the Daily Telegraph, News of the World, the Daily Express, and remains an associate editor of The Spectator.

Whilst freelancing between 1995 and 1997 Siôn was employed as a consultant to the Labour Party. In the 1997 General Election campaign, he ran the foreign press department at Labour Party Headquarters, having previously run the European desk in the 1992 election.

Siôn was elected to Parliament in June 2001 to represent the constituency of Birmingham, Erdington. He served on the Public Accounts Committee for two years from 2003, and was promoted to the position of Parliamentary Private Secretary to Foreign Office Minister Ian Pearson in 2005.

Siôn is a Trustee and board member of Action for Victims of Medical Accidents, and a founder and Trustee of the Choroideremia Research Foundation Inc.