The Honorary Born in Kilmun, Argyll, Scotland, McGregor worked in the Glasgow ship building yards prior to his emigration to the colony of South Australia in 1877. Initially working as a builder"s labourer and a gardener, McGregor became involved in the union movement, rising to the position of President and Secretary of the United Builders Labourers" Association and President of the Trades and Labour Council, which inevitably led to his involvement in the formation of the Labor Party, and served as President of the United Labor Party in 1893/94. He compensated for his blindness with a memory described as "astounding", able to recite lengthy passages perfectly after hearing them read, usually by his mother-in law.
As a Senator, McGregor served in Australian Labor Party government cabinets as Vice-President of the Executive Council prior to his death in 1914.
He served as the party"s first leader in the Senate and first Deputy Leader (at the time, the two posts were equivalent). McGregor married twice but had no children.