He represented Whitewood in the Legislative Assembly of the Northwest Territories from 1894 to 1904 as a Liberal-Conservative, and Whitewood (and then Pipestone) in the Legislative Assembly of Saskatchewan from 1905 to 1912 as a Provincial Rights (Conservative) member.

Gillis sat for Saskatchewan division in the Senate of Canada from 1921 to 1940.

[1] He was born in Whycocomagh, Nova Scotia, the son of Donald Gillis, a native of Scotland.

Gillis was Speaker of the Legislative Assembly of the Northwest Territories from 1901 to 1905.

He served as lieutenant-colonel in the 217th Battalion, Canadian Expeditionary Force during World War I.