 He was elected to the Legislative Assembly of Ontario in the 1959 provincial election.Singer became deputy leader of the Ontario Liberal Party and, as a Member of Provincial Parliament (MPP), pushed for reforms to municipal law, automobile insurance and electoral laws as well as for the establishment of a provincial ombudsman.He served as Liberal House Leader in the 1970s and, in 1973, sued fellow Liberal MPP Eddie Sargent for libel after Sargent made remarks about Singer's retainer fee from a developer while he was appearing before a legislative committee to explain how he obtained the contract to build Ontario Hydro's new headquarters in Toronto.