After graduating from Harvard University in 2001 with a Bachelor of Science in physics, Moulton joined the United States Marine Corps.

He served four tours in Iraq and then earned his master's degrees in business and public policy in a dual program at Harvard.

[15][14] Moulton was active in combat against insurgent forces in Iraq, including the 2004 Battle of Najaf against the militia of Muqtada al-Sadr.

[8] Between 2003 and 2008, Moulton was frequently interviewed about his experiences as an officer in Iraq by U.S. national media, including CNN, MSNBC, and NPR programs Morning Edition and All Things Considered.

The company raised investor funds and drafted a partnership agreement with Johns Hopkins School of Medicine, but in October 2014 the Boston Globe reported that by the time Moulton ran for Congress, EHP had no revenue, was still incubating, and had closed its only Massachusetts office.

[23][14] Moulton considered running against Democratic representative John F. Tierney of Massachusetts's 6th congressional district as an Independent in 2012, but decided against it in July 2012, saying, "the time and the logistics of putting together all the campaign infrastructure, organizing the volunteers ... the fundraising—it's just too much to accomplish in three months."

[35] In October 2014, he received criticism from his Republican opponent when he withdrew from a debate sponsored by radio station WGBH to instead attend a series of New York fundraisers hosted by Democratic National Committee chairwoman Debbie Wasserman Schultz.

[48] On August 24, 2021, Moulton, together with Republican representative Peter Meijer, flew unannounced into Hamid Karzai International Airport amid the evacuation of Americans and allies after the fall of Kabul.

[49] Moulton defended his Kabul trip, saying, "At the end of the day, I don’t care what pundits in Washington are saying...They’ve been wrong about this war for 20 years.

[60] This was beneficial for candidates like Abigail Spanberger who said, "[Moulton has] created a network of people who are able to lean on each other, share experiences, and discuss what the campaigns are like.

It raised the third largest amount of money among Democrats (after House majority leader Steny Hoyer and Senator Kamala Harris), according to campaign-finance non-profit Issue One.

[62] First-time candidates endorsed by Serve America who won their elections in the 2020 cycle included Mark Kelly from Arizona and Jake Auchincloss from Massachusetts’ 4th District.

"[77] According to The Boston Globe, "one of Moulton’s biggest focuses is addressing the long-term impact of automation on the economy, which he says will disproportionately affect working-class communities.

[79] In 2019, Moulton criticized Trump's withdrawal from the Trans-Pacific Partnership and said that, if elected president, he would reengage in negotiations for "a strong, fair trade deal for the Pacific on our terms, not China's".

[83] Moulton criticized President Joe Biden for the withdrawal of U.S. troops from Afghanistan, citing the chaotic evacuation of Afghans who had supported the U.S. in the effort to fight terrorism.

[84] He and Representative Don Bacon later co-sponsored the WELCOMED Act, to provide benefits and eligibility for resettlement to Afghan recipients of Special Immigrant Visas, which Biden signed into law on October 1, 2021.

Following Kamala Harris's loss to Donald Trump in the 2024 presidential election, Moulton expressed opposition to transgender participation in girls' sports.

[92] Moulton released a November 8 statement that included the following: I stand firmly in my belief for the need for competitive women’s sports to put limits on the participation of those with the unfair physical advantages that come with being born male.

Yet there are many who, shouting from the extreme left corners of social media, believe I have failed the unspoken Democratic Party purity test.

[97] On November 17, 2024, hundreds of people in Salem gathered at his office in the town at a rally held in protest at his remarks, co-organized by city councillor Kyle Davis.

[98] On November 25, 2024, Moulton said in an interview with the Rolling Stone that the media, in particular The New York Times, had misrepresented his position on transgender issues following the election.

[100] He also introduced the Brandon Act, a law intended to improve mental health access for active duty armed service members.

[101] In September 2018, Moulton, Elise Stefanik, and Dan Donovan co-sponsored the "Cyber Ready Workforce Act" advanced by Jacky Rosen.

It aims to offer certifications and connect participants with businesses to "boost the number" of workers for federal cybersecurity jobs.

[102] In January 2023, Moulton was one of 13 cosponsors of an amendment to the Constitution of the United States extending the right to vote to citizens 16 years of age or older.

[104] Moulton recently voted in favor of the Inflation Reduction Act which included historic funding into renewable energy innovation and implementation.

Moulton said: "Trump—a draft dodger who has mocked Senator John McCain, Gold Star families, and soldiers with traumatic brain injury—started talking about the good he has done for our military.

"[112] On June 15, 2016, Moulton appeared on the cover of the New York Daily News with the headline "No Civilian Should Own This Gun", in reference to semi-automatic assault weapons.

[113][114] Moulton penned an opinion piece promoting gun control, including the statement: "There's simply no reason for a civilian to own a military-style assault weapon.

[116] On June 23, 2017, Moulton announced his engagement to his girlfriend Liz Boardman, a senior client partner at an executive search firm.