After the war he studied as a lawyer and graduated from Osgoode Hall Law School in 1948.
He represented the ridings of York West and Humber in the west end of Toronto. He served as a cabinet minister in the government of Bill Davis. He worked with his father.
He was reeve of Etobicoke from 1963 to 1966 and was also chairman of the Etobicoke board of education.
In the 1971 provincial election he ran as the Progressive Conservative candidate in the riding of York West. He defeated Liberal candidate Dave Rattray by 14,180 votes.
He was re-elected in 1975 in the riding of Humber defeating Liberal candidate Alex Marchetti. He was re-elected in 1977.
MacBeth was appointed to cabinet On June 1, 1974 as Minister of Labour to replace Fern Guindon who was seeking Federal office.
In October 1975 he was promoted to Provincial Secretary for Justice and Solicitor General. In 1977, he briefly held the position of Minister of Correctional Services after Arthur Meen retired from office. He said, "I still get letters from people who are thankful that they do not have to work on those days." The legislation proved to be unwieldy and was eventually repealed in 1992.
He retired from politics in 1981.
After retiring from politics, he was appointed vice-chairman of the Ontario Police Commission which he held until 1987. He died while vacationing in Tulsa, Oklahoma and is buried at Park Lawn Cemetery in Etobicoke.
He was a Progressive Conservative member of the Legislative Assembly of Ontario from 1975 to 1981.