He was first elected to Parliament in 2001 and in his first term was the SNP Spokesperson on Trade and Industry and Work and Pensions, as well as being a member of the Scottish Affairs Select Committee (2001–05). In 2005, Weir held the new seat of Angus which had been substantially changed in the boundary changes for that election. Although he went into the election with a notional majority of only 532 he achieved a majority of 1601 over the Conservatives. Since 2005 Mike Weir has been the SNP Westminster Spokesperson on Business and Enterprise (which includes energy) and the Environment. He has been an outspoken opponent of nuclear power and has pursued a relentless campaign on the future of the Post Office network. Mike Weir sat on the Business, Innovation and Skills Select Committee (2005–09) and was a member of the Chairmen's Panel/Panel of Chairs from (2005–10). He served on the Energy and Climate Change Select Committee from 2009–10.