In January 1917, he was appointed to the Senate as a Conservative as a means of bringing in labour representation during the First World War.

When Prime Minister Sir Robert Borden formed a Unionist government in October as a means of creating a national government for the war effort, he included Robertson in the cabinet, making him minister without portfolio despite not having a seat in the House of Commons, to represent organised labour.

Robertson also supported the government's decision to send in the Royal North-West Mounted Police to crush the strike in an action that became known as "Bloody Saturday".

Robertson returned to government when R. B. Bennett's Conservatives won the 1930 election, and again became Minister of Labour, but remained unpopular with his constituents.

He stepped down as Labour minister in February and died in Ottawa the next year, two weeks after being paralyzed by a stroke.