[6][7] In his second year of law school, Eldridge managed then-State Representative Pam Resor’s successful re-election campaign in 1998.

[3][4] He worked as a prosecutor in the towns of Natick and Framingham, and assisted on the district attorney's community-based justice program in the City of Lowell.

[3][4] In his final year of law school, Eldridge was elected chair of the Acton Democratic Town Committee.

[3][4] He worked as a public interest lawyer in the fields of housing, Social Security disability, and unemployment law.

[6] Eldridge left his fellowship early to devote his full energies to running for the 37th Middlesex District open seat.

[6] In 2007, Eldridge was a candidate in the Democratic primary for a special election to replace Marty Meehan as United States Congressman for Massachusetts's 5th congressional district.

[4] Eldridge assumed the Massachusetts Senate seat formerly held by Pam Resor[12] (District of Middlesex and Worcester) on January 7, 2009.

[12] In 2020, he endorsed Bernie Sanders in the Democratic Party presidential primaries, 2020 and later served on his campaign as state co-chair.

[20] Eldridge is an Acton, Massachusetts, native and the son of a public school kindergarten teacher and electrical engineer.