Martin Smyth was born in Belfast and educated at Methodist College Belfast, and Trinity College Dublin before going onto to train as a Presbyterian Minister at Magee University College, Londonderry, and the Presbyterian College, Belfast. After his ordination in 1957 he served in a number of churches in the Greater Belfast area. Smyth's first involvement in politics came in the 1960s when he joined the Ulster Unionist Party (UUP) and he went onto become a member of its ruling body, the Ulster Unionist Council (UUC). He later served on its executive including spells as Chairman and Vice President (1972-2000). In May 1975 he was elected as a member of the UUP representing the constituency of Belfast South in the Constitutional Convention (1975-77). During this period he was a member of a UUP delegation which met with members of the Social Democratic and Labour Party (SDLP) in secret discussions which attempted to end the political stalemate at that time. When the Convention was wound up in 1977 he returned to his work as a Presbyterian Minister as well as maintaining his association with the Orange Order in Ireland. He had joined the Order as a young boy and had gradually moved through the ranks to assume the position of Grand Master of the Grand Orange Lodge of Ireland (1973-82 and 1993-2000).
Following his election in February 1982 as MP for the Westminster constituency of Belfast South (1982-present) he gave up his church ministry to become a full-time politician. In October 1982 he was also returned as a representative of Belfast South in the Northern Ireland Assembly of 1982-86. With the signing of the 1985 Anglo-Irish Agreement (AIA) he participated in the unionist campaign against it. Smyth also resigned his parliamentary seat in January 1986 in order to use the subsequent by-elections to highlight the extent of the opposition within unionism to the AIA. During the period 1991 to 1992 he was a member of the UUP's delegation to the all-party talks which were later to end without success.
During political developments, in what was to become the 'Peace Process' in the early to mid 1990s, Smyth did express a willingness to consider the possibility of the UUP dealing with republicans but he stressed this could only be done under certain terms. When the Good Friday Agreement (GFA) was signed in April 1998 he felt these terms had not been met and he immediately became associated with the anti-GFA element within the UUP. This led to him campaigning for a 'No' vote in the May 1998 referendum. Since then he has remained a prominent critic of the policies of his party leader, David Trimble, concerning the implementation of the GFA and in particular the move to participate with republicans in the Northern Ireland Executive. In March 2000 this led to him challenging Trimble for the leadership of the UUP at a meeting of the UUC and although he was to fail, Smyth did poll well and secured over 43% of the votes cast. In spite of this personal setback he has continued to oppose the GFA and to advocate that the UUP should pull out of the Executive in the absence of the complete decommissioning by republican paramilitary groups. He continued however to voice criticism of his party leader, David Trimble, and this has taken different forms. For instance aong with two of his fellow Westminster MPs, Smyth resigned from the party's group at Westminster from June 2003 until October 2003.