He served in the Legislative Assembly of Alberta from 1967 to 1982 as a member of the Social Credit caucus both in government and in opposition.
Mandeville improved his margin of victory in the new electoral district to pick it up for Social Credit who became the official opposition after the Progressive Conservatives formed government.
[4] In the 1979 general election Mandeville won the largest share of votes of his career as he defeated three other candidates to keep his seat.
[5] During the session, former leader Robert Curtis Clark resigned, and two others, Raymond Speaker and Walt Buck, left the party to sit as independents shortly after the call of the 1982 general election, leaving Mandeville as the sole member of the once-proud party's caucus.
Mandeville retired from the legislature at dissolution in 1982; he was the last person to sit in the Alberta Legislative Assembly under the Social Credit banner.