He said that "well over two-thirds" of the board members required to back Markey for the group's endorsement had voted in favor of him.
Markey's primary opponent, Rep. Stephen Lynch (D-Mass.), had been the favorite of unions in Massachusetts, and the AFL-CIO board was split between the two during the primary, which resulted in the group sitting out the race.
Earlier reports indicated the AFL-CIO was working to shore up the needed support to endorse Markey, as some unions indicated they were dissatisfied with the Democratic Party.
“We did not endorse in the primary, and we’re extremely disappointed on our return on investment with Democrats,” local IBEW President Mike Monahan told the Boston Herald, adding that his group planned to talk to Republican nominee Gabriel Gomez.
The AFL-CIO also endorsed Sen. Elizabeth Warren (D-Mass.) during her 2012 race, and the group's support will give Markey the manpower of one of the state's most poweful unions, for canvassing and organizing purposes.