Thériault named Byrne to his cabinet in the roles of Minister of Justice and Attorney General of New Brunswick, and Government House Leader.

Like many of his Liberal colleagues, Byrne fell victim to the Progressive Conservative sweep in the 1999 election, losing his seat by only 264 votes.

Byrne ran and won the riding of Fredericton-Lincoln running as a Liberal candidate in the 2006 election.

Following the election, he stepped down as president of the party and was returned to the cabinet as Minister of Business New Brunswick.

Byrne failed to regain his seat in the general election of 2010, when the Graham government was defeated by David Alward and the Progressive Conservative Party.