Mullin joined the SNP in 1966. He is a former Vice-convener of the Scottish National Party. He was the SNP candidate at the Paisley North by-election in 1990, where he finished in second place behind Irene Adams of the Labour Party, with 29.4% of the vote. He also finished in second place at the 1992 general election in the same seat, with a reduced share of 23.3% of the vote. He unsuccessfully fought three earlier Westminster campaigns at the February 1974 general election in South Ayrshire, again in South Ayrshire at the October 1974 general election and at Kirkcaldy in 1987. His wife, Barbara Mullin, was also a candidate for the SNP at the 1992 general election in the Ayr constituency. However, neither him or his wife were elected. In January 2015, Mullin was selected as the official SNP candidate for the Kirkcaldy and Cowdenbeath constituency and subsequently was elected on 7 May 2015 with 27,628 votes and a 52.2% of the vote. Mullin had a majority of 9,974 votes over Kenny Selbie, the Labour Party candidate who was hoping to succeed Gordon Brown, the former Prime Minister who retired in what had once been Labour's safest seat in Scotland. Kirkcaldy and Cowdenbeath saw a turnout of 69.73%; the highest recorded for this constituency. As a Member of Parliament, Mullin was a Treasury Spokesperson for the SNP, and a member of the Regulatory Reform Select Committee He led a campaign in Westminster to reform Scottish Limited Partnerships which had been exposed by the Herald newspaper in Scotland as being vehicles for tax evasion and laundering of criminal assets. Since leaving parliament he has continued to comment and campaign on Scottish Limited Partnerships, including being critical of the Law Society of Scotland's lack of effective action. Mullin presented a ten-minute rule bill to parliament on 20 April 2016 entitled Forensic Linguistic on 13 October 2015. Mullin led a Westminster Hall debate on the subject of The Use of Children as Suicide Bombers spelling out the significant rise on the use of children and young girls as human guided weapons. Mullin was Chair of the All Party Parliamentary Group for Explosive Weapons, in which capacity he addressed the 19th International Meeting of Mine Action National Programme Directors and United Nations Advisers at the United Nations in Geneva 2016. He led a Westminster Foundation for Democracy project in Iraqi Kurdistan, and while in Kurdistan, became the first British MP to enter the City of Mosul during hostilities to see at first hand work being undertaken to begin to clear the city of improvised explosive devices. In 2016, he was appointed by the SNP leader and Scottish First Minister Nicola Sturgeon to the party's Scottish Growth Commission. After leaving Parliament, Mullin was appointed Special Envoy for the All Party Parliamentary Group on Explosive Threats in July 2017. He has also undertaken research into Brexit and Scottish Business and initial research into a stock exchange for Scotland along with former MP colleague Michelle Thomson. In May 2018, Mullin became part of the Advisory Team of the African Entrepreneurial Network
Publications
Scottish Business and International Trade - Perceptions of the Scottish Business Diaspora, Momentous Change Ltd, January 2020
Organization Development: How Organisations Change and Develop Effectively, London, 2020
Trust in UK Banking. International Banker. September 2019.
A Stock Exchange for Scotland: Early background research, Momentous Change Ltd, November 2018.
Facing the Brexit Challenge. iScot Magazine, May 2018.
The Changing Face of Suicide Bombing. Cable, Issue 2, August 2017
Public and Third Sector Leadership, London, 2014
Mhairi's Dilemma: A study of decision analysis at work. Judgment and Decision Making, Vol. 3, No. 8, December 2008, pp. 679–689
Career Goals and Educational Attainment: What is the Link?. Careers Scotland, 2004
An Evaluation of the Higher Still Reforms. Insight, No. 11, The Scottish Executive, 2003
Lifelong Learning: A Radical First Step. Holyrood, Issue 68, April 2002
The Role of FE in Economic Development, Broadcast no.42, SFEU, 1998
Good Judgment. Policing, vol ii, no. 4, Winter 1995, pp. 272–281
The Competence Debate. Training and Development. London, 1993.