Two years into the job, Labor Secretary Marty Walsh is joining the Great Resignation.

The Labor Department announced Thursday that Walsh, a former union leader and mayor of Boston, will leave his post in mid-March. His next stop: the National Hockey League Players' Association, where he was unanimously appointed Executive Director, the NHLPA said in a statement.

"As someone who grew up in an active union family and is a card-carrying union member, serving as Secretary of Labor and being given this unique opportunity to help working people is itself a privilege," Walsh said in a letter to colleagues shared by the Labor Department.

He called Biden "the most pro-worker and pro-union president" in U.S. history.

Walsh's Senate confirmation in March 2021 was celebrated by labor organizations and unions who were thrilled to see one of their own installed as Labor Secretary.

In what was perhaps his biggest test as Labor Secretary, Walsh stepped into the high-profile labor dispute between the nation's freight railways and the rail unions, brokering a tentative deal to avert a nationwide rail strike. However, the deal proved unpopular with rank-and-file rail workers for its lack of paid sick leave, among other things. Some rail workers blamed Walsh, saying he, along with Biden, had let them down.

In the end, after multiple rail unions voted to reject the deal, Congress stepped in to impose the terms to keep the trains running through the holidays. Shortly thereafter, one freight railroad reopened talks with unions over providing paid sick leave, announcing deals earlier this month.

Under Walsh's leadership, the Labor Department has pushed for a reshaping of workplace laws and regulations, including proposing a rule that would lower the bar for who must be classified as a employee of a company rather than an independent contractor. The rule could affect construction workers, home health care aides, custodians and others who, as independent contractors, are not entitled to overtime pay and other federal protections.

"While independent contractors have an important role in our economy, we have seen in many cases that employers misclassify their employees as independent contractors, particularly among our nation's most vulnerable workers," Walsh said last October, when the proposed rule was unveiled.

The son of Irish immigrants, Walsh grew up in the working-class Dorchester neighborhood of Boston and followed his dad into construction, helping to build Boston's waterfront. He rose to lead Laborer's Local 223 and later the umbrella organization known as North America's Building Trades Unions, where he represented tens of thousands of construction workers.

As news of Walsh's departure emerged, labor groups offered praise.

"Marty Walsh has labor in his bones, and he proudly championed the nation's workers in Washington just as he's done throughout his life and career," said American Federation of Teachers President Randi Weingarten. "North America's hockey pros, Boston Bruins players among them, could not ask for a more dedicated and committed advocate."

In his goodbye letter, Walsh praised his deputy Julie Su, who formerly led California's labor and workforce agency, saying he was "confident there will be continuity and the work will be sustained."