Sillars was born in Ayr, the son of Matthew Sillars, a railwayman, and Agnes Sillars, a carpet weaver. He was educated at Newton Park School and Ayr Academy. After leaving school he worked as an apprentice plasterer, before following his father into working on the railways. Sillars served as a radio operator in the Royal Navy from 1956 to 1960, before becoming a Firefighter. It was as a fireman that he became more active politically, through the Fire Brigades Union, and he joined the Labour Party in 1960. He served as a member of Ayr Town Council from 1962 to 1970, and was Head of Organisation and Social Services at the Scottish Trades Union Congress from 1968 to 1970.
In 1976 he led a breakaway Scottish Labour Party. The formation of the SLP was inspired primarily by the failure of the then Labour Government to secure a Scottish Assembly. Sillars threw himself into establishing the SLP as a political force, but ultimately it collapsed following the 1979 General Election. At that election the SLP had nominated a mere three candidates. Only Sillars came remotely close to winning and it was this failure to secure a meaningful share of the vote that prompted the decision to disband.
In the early-1980s, Sillars joined the Scottish National Party. Being a left-winger he had fostered close links with the SNP internal 79 Group; who had encouraged him to join. Sillars, along with the 79 Group and the former SLP members in the SNP, started to shape the SNP as a clearly defined, left-of-centre party. Policies adopted included the support of a non-payment scheme in relation to the poll tax introduced by the Conservative government of Margaret Thatcher, as well as the policy of independence within the European Union, of which Sillars was a leading exponent. Sillars also started talking in terms of direct action to bring prominence to the Scottish independence cause, stating that: "we must be prepared to hear the sound of cell doors slamming behind us if we are prepared to win independence". Having failed to win the Linlithgow seat from Tam Dalyell of the Labour Party at the 1987 general election, Sillars was chosen to be the SNP candidate for the Glasgow Govan by-election. Govan was a Labour seat, but Sillars won a dramatic victory. Sillars became the SNP's Deputy Leader, with many surprised he did not stand for the party leadership when it became available in 1990. The 1992 general election proved a disappointment for Sillars personally; as he lost his Glasgow Govan seat. It was at this time that Sillars made his famous comment that the Scottish people were "90 minute patriots". This comment proved the beginning of a break with the SNP leadership. The SNP leader at the time, Alex Salmond, had been a Sillars ally, but his comments in the aftermath of the 1992 general election, started this break. In 2016, contrary to the SNP position, he announced he would campaign in favour of British withdrawal from the EU during the 2016 United Kingdom European Union membership referendum. He has said: "I think is a profoundly undemocratic organisation which has shown a callous disregard for people, in Portugal, Spain and Greece for example. They've been willing to make people destitute - beggar nations - in pursuit of a single policy to create a United States of Europe irrespective of whether the people want it."
Sillars said during the 2014 Scottish independence referendum that “BP, in an independent Scotland, will need to learn the meaning of nationalization, in part or in whole, as it has in other countries who have not been as soft as we have forced to be. We will be the masters of the oil fields, not BP or any other of the majors.”
