Seddon became editor of Tribune in 1993, a job he kept until 2004. He was elected to the National Executive Committee of the Labour Party as a Grassroots Alliance candidate in 1997, gaining the highest share of the vote, and remained an NEC member until 2005.
Seddon tried to find a parliamentary seat and stood in the safe Conservative seat of Buckingham in the 2001 general election against John Bercow. In 2002, he was controversially removed from the shortlist to be Labour's candidate in the Ogmore by-election, a process that was repeated in 2010 when he sought selection for the Labour seat of Stoke-on-Trent Central. During the 1992 general election, he worked for Gordon Brown and served for five years on the Chancellor of the Exchequer's Economic Policy Commission.
Seddon was a vocal critic of the last Labour government, particularly over the 2003 invasion of Iraq. He backed Mayor of London Ken Livingstone's ultimately successful attempts to be readmitted to the Labour Party. His book, Standing for Something, is about his time as a dissenter within New Labour.
Since the beginning of 2011 Seddon has been director of People's Pledge, a political campaign that seeks a referendum in the United Kingdom on the European Union.