Prowse was introduced to politics at a young age when his father, James Harper Prowse Sr., ran as a Conservative in the
1926 provincial election in the electoral district of
Taber. Prowse first ran for a seat in the legislature in the
1944/1945 armed forces vote that was the last stage of the 1944 general election. He ran as a candidate for the army seat and on February 5, 1945 won the seat with just 17 percent of the vote, defeating 21 other candidates. The vote was non-partisan so Prowse sat as an independent in the legislature. After winning the election and returning to Edmonton, Prowse worked as a journalist for the
Edmonton Bulletin. He crossed the floor to the Liberals after announcing his intention to run for the leadership of the party on April 10, 1947. He said of his decision, "The political situation has reached a point where there is no longer any advantage to be gained by remaining neutral." Prowse was elected leader of the party on the first ballot at the Liberals' annual convention on June 26, 1947. The armed forces seats were abolished after the Second World War, and were not be filled in the 1948 election. Prowse ran for a seat in the
Edmonton electoral district in the
1948 Alberta election. He took the fourth of five seats in the multi-member district, the seats were allocated using the
single transferable voting election system. With the use of proportional representation in Alberta's two largest cities, the Liberal party received 17 percent of the popular vote province-wide and won one seat in Edmonton, one in Calgary, and none in the districts where
instant-runoff voting was used. In the
1952 Alberta general election Prowse won one of the seven seats in the Edmonton electoral district. He led the Liberals to four seats and 22% of the popular vote. In the
1955 general election the Liberals made their best showing in decades, winning 15 seats and earning 31 percent of the popular vote. Prowse again took the second seat in Edmonton. Prowse stepped down as leader of the Liberal party in 1958 and retired from the legislature at dissolution in 1959. He did not run in the 1959 provincial election. He ran for mayor of
Edmonton in the
1959 municipal election; he lost to
Elmer Roper. ==Federal politics==