Robertson was a toolmaker and engineer and was assistant divisional organiser of the Amalgamated Engineering Union 1954-1961 and secretary of the Scottish Iron and Steel Trades Joint Committee. He served as a councillor on Lanarkshire County Council and Motherwell and Wishaw Council from 1946. At the 1951 general election Robertson stood as the Labour candidate in the marginal Conservative-held seat of Glasgow Scotstoun, losing by 625 votes.
He was elected to the House of Commons ten years later at the 1961 by-election in the Paisley constituency, following the appointment of sitting Member of Parliament Douglas Johnston as a judge in the Court of Session.
He was re-elected at five subsequent general elections, sitting as a Labour Member of Parliament until 1976, and for the SLP between 1976 and 1979. He did not contest the 1979 general election.
On 18 January 1976 Robertson, along with another Labour Member of Parliament, Jim Sillars and Labour"s senior Scottish researcher, Alex Neil, officially established the Scottish Labour Party (SLP). The formation of the SLP was prompted as a left-wing split from the mainstream Labour Party over distrust about how strong the then Labour government"s stated commitment to Scottish devolution was in reality.
Robertson was a leading figure in the SLP, along with Sillars, but unlike Sillars he stood down at the 1979 general election, deciding not to contest his Paisley seat.