[4][5] Moon has worked for various activist groups, including as the chief operating officer of FairVote from 2004 to 2009 and as the program director for Demand Progress since 2011.

[7] In 2013, Moon launched a campaign for the Maryland House of Delegates,[8] seeking to succeed Heather Mizeur and Tom Hucker, both of whom had sought other office in 2014.

The Montgomery County Democratic Central Committee voted 19–8 to nominate delegate William C. Smith Jr. to fill the vacancy.

[24] In 2017, Moon led efforts to pressure the Hogan administration to implement tougher water pollution regulations on the state's coal-fired power plants.

[26] In September 2022, Moon authored a letter in support of a proposed U.S. Energy Department rule that would increase the efficiency level of gas furnaces to 95 percent.

[27] During the 2018 legislative session, following the 2017 Las Vegas shooting that left 60 dead and about 867 injured, Moon introduced a bill to ban bump stocks.

[30] During the 2023 legislative session, Moon introduced a bill to audit the state's behavioral health and drug treatment system to identify ways to support community-based preventative services.

[42] In September 2018, Moon signed a letter calling for a county investigation into sexual assault allegations made against U.S. Supreme Court nominee Brett Kavanaugh.

[43][44] Montgomery County law enforcement officials declined to investigate the matter unless the alleged victim filed a complaint.

[45] While serving as a program director for Demand Progress, Moon led protests against the Stop Online Piracy Act and related bills.

[48] During the 2018 legislative session, Moon introduced a bill that would prevent the Washington Redskins from receiving subsidies to build a new stadium unless the team changed its name.

[51] In January 2019, Moon was one of nine Maryland lawmakers to add their names to a manifesto signed by 326 state legislators to reaffirm their commitment to protecting abortion rights.

The bill was withdrawn after House Speaker Michael E. Busch and Senate President Thomas V. Miller Jr. said they would begin livestreaming sessions in 2020.