He represented Lunenburg from 1917 to 1925, Queens—Lunenburg from 1925 to 1926 and Antigonish—Guysborough from 1927 to 1936 in the House of Commons of Canada as a Liberal member.

Duff represented Lunenburg division in the Senate of Canada from 1936 until his death in 1953.

He settled at Bridgewater, Nova Scotia, where he married Jennie E. Oxner.

He was defeated in a bid for reelection in 1926 in Queens—Lunenburg and then was elected in Antigonish—Guysborough in a by-election held later that year following the death of John C. Douglas.

In 1953, Duff died of a heart attack at home in Lunenburg while still a member of the Senate.