Seth Wilbur Moulton was born on October 24, 1978, in Salem, Massachusetts, to Lynn Alice Meader and Wilbur Thomas Moulton Jr., a real-estate attorney. He grew up in Marblehead and graduated from Phillips Academy in 1997. He earned a bachelor's degree in physics from Harvard College in 2001, then served as a Captain in the U.S. Marine Corps, completing four tours in Iraq as a combat infantry officer, where he was twice decorated for heroism. After his military service, Moulton earned master's degrees in business administration and public policy from Harvard in a dual program. He was first elected to Congress in 2014, representing Massachusetts's 6th congressional district since 2015, which includes much of Boston's northern suburbs such as Andover, Marblehead, Peabody, and Salem. He ran briefly for president in 2019 before withdrawing after failing to qualify for debates. At age 47, Moulton presents himself as a generational alternative to the 79-year-old incumbent Markey, emphasizing his military service, combat experience, and centrist-moderate Democratic positioning.

Moulton is known as a provocative and outspoken member of Congress who challenges Democratic Party establishment, having led an unsuccessful 2018 effort to remove Nancy Pelosi as House Speaker and calling for generational leadership. He sits on the Armed Services Committee and Transportation and Infrastructure Committee. Since launching his 2026 Senate campaign in October 2025, he has emphasized generational change, affordability, healthcare, national security, and Democratic Party reform.