His father worked as a janitor at Regis College and had a side landscaping business, and his mother was a nurse's aide.

[1] Walsh's political career began shortly after he graduated from college when he helped a friend who was running for the Abington School Committee.

Walsh served as Deval Patrick's campaign manager during his successful 2006 Massachusetts gubernatorial run[2][5] and was also the director of his transition team.

During his tenure as chairman, Democrat Martha Coakley was upset by Republican Scott Brown in the 2010 United States Senate special election.

After this loss, Walsh oversaw the party's grassroots campaign that helped Senate candidates Elizabeth Warren and Edward Markey win in 2012 and 2013, respectively.

[11] In the fall of 2018, Walsh was a founder and the treasurer of Reason To Believe PAC, an organization established to support progressive Democrats and grassroots political efforts nationwide in the 2018 election cycle.