Thériault was first elected to the Legislative Assembly of New Brunswick in the 1987 provincial election that saw the Liberal Party, of which he was a member, win every seat in the province. Due to the unusual situation of leading a government with no parliamentary opposition, PremierFrank McKenna named backbench members of his caucus to form a shadow cabinet. Thériault was the leader of this "unofficial opposition", which met daily when the house was in session to prepare questions of Question Period of which the ministers would be given no notice. Following the 1991 election, some balance was restored to the legislature with the opposition parties holding 12 of 58 seats and the "unofficial opposition" was not continued. Thériault was named to cabinet as Minister of Fisheries. In 1994, he was appointed Minister of Advanced Education and Labour Thériault was re-elected in 1995 and became Minister of Economic Development—a key role in the McKenna government. When McKenna announced his resignation in 1997, Thériault, who had long expected to be the favourite, saw the early position of front-runner fall to charismatic Finance MinisterEdmond Blanchard. Blanchard soon dropped out of the race. Thériault left cabinet to campaign, as did his two opponents, Education Minister Bernard Richard and junior cabinet ministerGreg Byrne. Byrne, a relative unknown before the race, caught momentum during the race but Thériault managed to win on the first ballot. As leader of the governing Liberals, he soon became premier. He rejigged the cabinet upon being sworn-in, changing some departments and decreasing its overall size. He pledged to bring in many new programs, focussing on social services following the fiscal conservatism of McKenna, but felt he should win a mandate of his own before instituting any major changes. The OppositionProgressive Conservatives had themselves just chosen a leader, the young Bernard Lord, an unknown with no political experience. As has always been Canadian tradition, Thériault waited for Lord to contest a seat for the legislature and get some experience in the House before calling a vote. Despite that, Thériault gave up one of the key advantages of the incumbency, letting it be widely known months in advance that the election would be held in June 1999. Thériault and his Liberals widely underestimated Lord's Conservatives over whom they enjoyed a double-digit lead inopinion polls. Early in the campaign, Lord reversed his earlier position, shared with the government, supporting highway tolls on the new divided route from Fredericton to Moncton. Lord used his new pledge to remove the tolls as the centrepiece of his campaign, he effectively used the issue as the prime example for the arrogance of the government and also incorporated his pledge into his "200 Days of Change" promise—20 key commitments Lord said he would implement within his first 200 days in office. Thériault largely ignored Lord's surging campaign as he felt the Liberals would easily cruise to victory—a feeling mirrored in polls right up until the last week. On June 7, election, the Tories won their largest victory ever taking 44 of 55 seats. Thériault's concession speech was very gracious and probably his best of the campaign, he said "the people have spoken, and the people are never wrong" in seeing the party's seats reduced from 45 to 10. Thériault stayed on as leader of the opposition and was very effective in the role. Despite this, the Liberals lost two by-elections in early 2001 that had been vacated by former Liberal cabinet ministers moving on to federal government. In March, Thériault resigned his seat and his leadership—both effective immediately. Following his resignation, Prime MinisterJean Chrétien appointed him to the federal Transportation Safety Board, within the year he was named its chairman. In 2004, he was named president of the Mouvement des caisses populaires acadiennes. Thériault mused briefly about running in the 2004 federal election but opted not to.
