Robert Mercer Johnston (September 15, 1916 - October 16, 1985) was an Ontario political figure.
He represented St. Catharines in the Legislative Assembly of Ontario from 1967 to 1977 as a Progressive Conservative member.
[1] Johnston was elected mayor of St. Catharines, Ontario in 1964 defeating incumbent Ivan Buchanan.
[2] He remained mayor until he was elected to provincial office in 1967.
He defeated Liberal candidate Jim Bradley by 3,948 votes.