He was appointed as King's Counsel in 1922.He was first elected to Parliament at the Westmount—St.
Henri riding in the 1921 general election.
After riding boundary changes in 1924, Mercier was re-elected at the new St.
Henri electoral district in the 1925 federal election, then re-elected there in 1926, 1930 and 1935 (in 1933, the riding's English name became "St.
Henry").
On 29 November 1937, Mercier accepted an appointment to become a Montreal region Circuit Court judge, leaving the House of Commons before completing his term in the 18th Canadian Parliament.