In signing a party card on Thursday in Fredericton, Kelly Lamrock said he has no intention of running as a candidate in next year's provincial election.
Instead, Lamrock will help the party develop new policies and its election platform.
"I'm looking forward to running the platform process to show that we can be open and substantive and not just stay quiet and try to win the election by being a little less objectionable than the other guy," Lamrock said.
Lamrock was first elected in 2003 and was named to cabinet after the Liberals became the government in 2006. He served in the portfolios of education, social development and attorney general before being defeated in the 2010 election.
Lamrock said he wants to continue to contribute to politics but the Liberal party under new leader Brian Gallant is moving to the right.
"I didn't leave the Liberal party — the Liberal party chose to leave me," he said.
Gallant said Lamrock served the Liberal party well.
"We are happy to see his continued involvement in provincial politics, and wish him all the best in his future endeavours," Gallant said in a statement.
Lamrock said the New Democrats embrace the direction he wants to see, such as taking politics out of economic development and protecting social policies.
He will also be involved in the launch of a new website intended to explain NDP policies and seek input.
NDP Leader Dominic Cardy said he has been speaking to members of all parties and expects to have more people join his team before the next election.
"We have to build a unity party in New Brunswick that brings together Liberals, Conservatives, New Democrats and people from other parties who share a common commitment to wanting to lift kids out of poverty, making sure our government is run efficiently and effectively, and having the best run public service," he said.
The NDP have no seats in the legislature. The Progressive Conservatives have 41, the Liberals have 13 and there is one Independent.