He was first elected to the Manitoba legislature in the 1903 provincial election, defeating Liberal candidate F.W. Clinigan by 25 votes in the Virden constituency. He was appointed to Roblin's cabinet on March 2, 1904 as Provincial Treasurer. 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.
He was re-elected with an increased majority in the 1907 election, and was retained as Provincial Treasurer in the legislative sitting that following. He died in office in 1908.