In the late 1960s he became a MP for the Labour Party, representing the South Ayrshire constituency. He became well known as an articulate, intellectual left-winger, strongly in favour of the establishment of a devolved Scottish Assembly.
In 1976 he led a breakaway Scottish Labour Party (SLP). The formation of the SLP was inspired primarily by the failure of the then Labour Government's failure to secure a Scottish Assembly. Sillars threw himself into establishing the SLP as a political force, but ultimately it would collapse following the 1979 General Election. At that election the SLP had nominated a mere three candidates (including Sillars who was attempting to hold on to his Ayrshire seat). However only Sillars came remotely close to winning and it was this failure to secure any real vote that prompted the decision to disband.
In the early 1980s Sillars (along with many other former SLP members) joined the Scottish National Party (SNP). 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 Tory Government of Margaret Thatcher, as well as the policy of independendce in Europe, in 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'.
In 1988 Sillars was chosen as the SNP candidate for the Govan by-election. Govan was a Labour seat (although the SNP had won it in a by-election previously, in 1973), but Sillars proved an inspired choice. His sound use or oratry and his street campaigning style brought life to the SNP and they won a dramatic victory.
Sillars would become the SNP's deputy leader, with many surprised he didn't stand for the party leadership when it became available in 1990. The 1992 General Election proved a disappointing one for Sillars personally as he lost his Govan seat. It was at this time that Sillars made his famous comment that the Scottish people were '90 minute patriots' (a reference to the amount of time a Football/Soccer match lasts).
This comment started to prove the beginning of a break with the SNP leadership. The then SNP leader Alex Salmond had been a Sillars ally, but his comments in the aftermath of the 1992 General Election (and it is also suspected the fact that Sillars supported Salmond's leadership contest opponent, Margaret Ewing) started this break.
Sillars remains a columnist for the Sun Newspaper and he has used his column to criticise what he sees as poor political strategy from the SNP leadership, first under Salmond, and latterly under John Swinney. Sillars is viewed as belonging to the SNP Fundamentalist camp.