He served in the Legislative Assembly of Manitoba from 1903 until his death as a member of the Conservative Party, and was a prominent cabinet minister in the government of Rodmond Roblin.
Agnew was born in Prince Albert, Ontario County, Canada West (now Ontario), the son of Dr. Niven Agnew and Jane Somerville Ross, and was educated at Upper Canada College.
Agnew moved to Winnipeg with his family in 1879; he studied law, articling with James Albert Manning Aikins, and was called to the Manitoba bar in 1884.
New cabinet appointees were required to seek a renewed mandate from their electors in this period of Canadian history, and Agnew was returned for Virden without opposition on March 12.
[2] He was re-elected with an increased majority in the 1907 election, and was retained as Provincial Treasurer in the legislative sitting that following.