On February 29, 2024, President Biden nominated Walsh to serve as a governor of the USPS, for a term that would have expired on December 8, 2029. On November 14 (as a lame-duck president following the 2024 election), Biden withdrew the nomination from Senate consideration without providing an immediate explanation.

The 29th United States secretary of labor from 2021 to 2023, Walsh was the first former union leader to serve in that position in roughly 45 years. Was also the first-ever Cabinet member to openly be in a twelve-step program for recovery from addiction.

The 53rd mayor of Boston from 2014 to 2021.

In late-2010, Walsh was elected to serve as both the secretary-treasurer and general agent of the Boston Building Trades Council from 2011 until 2013, representing 35,000 union construction workers in the Boston area.

Walsh joined Laborers' Union Local 223, at the age of 21 in 1988. In 2001 he became its secretary, and in 2005 he became President of Laborers' Union Local 223.