First elected to the Saint John City Council in 1971 he went into provincial politics in 1972, winning a by-election for the Provincial Progressive Conservative Party. Re-elected to the Legislative Assembly in 1974, he would be appointed to the Cabinet as Minister of Education then in 1976, Minister of Commerce & Development. After winning re-election again in 1978 and in 1982, he became the Minister of Natural Resources, Government House Leader, and Deputy Premier.

Gerald Merrithew resigned his provincial seat to run as a Progressive Conservative Party of Canada candidate for the Saint John riding in the 1984 Canadian federal election. Elected to the House of Commons of Canada in Ottawa, and with his party winning the election, Merrithew was immediately appointed Minister of State for Forestry then in 1986 to Forestry & Mines. After winning his seventh consecutive election in 1988, he was appointed Minister for the purpose of the Atlantic Canada Opportunities Agency and Minister of Veterans Affairs.^{[citation needed]}

In 1993, Merrithew resigned from Cabinet and did not seek re-election. He and his wife retired to their farm in Springfield, New Brunswick at the head of the Belleisle Bay.^{[citation needed]}

Active with numerous Military and Veterans Associations, Merrithew was a member of the Queen's Privy Council for Canada and also held several directorships including the "26th Battalion Overseas Association Inc." where in 1995 he was instrumental in publishing the history of the Battalion by S. Douglas MacGowan (ISBN 1896270026).

Gerald Merrithew died of cancer on 5 September 2004, aged 72.^{[citation needed]}

• Hansard, 1st Session, 38th Parliament, Volume 142, Issue 16
• Gerald Merrithew – Parliament of Canada biography
• Legislative Assembly of New Brunswick tribute to Gerald Merrithew