Prior to being elected to the Assembly in 2014, Chiu was the President of the San Francisco Board of Supervisors and Supervisor for District 3, encompassing the northeastern corner of the city, including Chinatown, North Beach, the Financial District, and Fisherman's Wharf.
Chiu is a member of the California Asian & Pacific Islander Legislative Caucus. He was the first Asian American to serve as District 3 supervisor, the first Asian American to serve as board president, and the first supervisor elected to three consecutive terms as board president.
The eldest child of Hakka Taiwanese American immigrant parents, Chiu was born in Cleveland, Ohio and grew up in Boston, Massachusetts, where he attended Boston College High School. Chiu graduated from Harvard University, where he received a bachelor's degree in government from Harvard College, a master's degree in public policy from Harvard’s Kennedy School of Government, and a Juris Doctor degree from Harvard Law School.
In the mid-1990s, Chiu served as Democratic Counsel to the U.S. Senate Judiciary Committee’s Constitution Subcommittee and aide to Senator Paul Simon to the Senate Budget Committee. After moving to San Francisco in 1996, Chiu worked as judicial clerk, as a criminal prosecutor at the San Francisco District Attorney’s office, and as a civil rights attorney at the Lawyers' Committee for Civil Rights. He later founded Grassroots Enterprise, an online communications technology company, and served as its chief operating officer. He also served on the San Francisco Small Business Commission until he was elected supervisor in 2008.
Chiu first ran for elected office in 2008, when he ran for a seat on the San Francisco Board of Supervisors representing District 3. He was backed by incumbent supervisor Aaron Peskin as well as Kamala Harris, Mark Leno, Leland Yee, and Tom Ammiano.
On his first day in office on January 8, 2009, Chiu was elected to a two-year term as president of the Board of Supervisors. He was reelected president on January 8, 2011.
Chiu was reelected to his second and final term as supervisor in 2012, winning over 75% of the vote. He was later reelected by his fellow supervisors to serve an unprecedented third term as president of the board on January 8, 2013. In addition to serving on the Board of Supervisors, Chiu also served as a member of the San Francisco Democratic County Central Committee.
On January 10, 2011, Chiu served as acting Mayor of San Francisco for a day after Gavin Newsom was sworn in as Lieutenant Governor of California, prior to the selection and swearing-in of Ed Lee by the Board of Supervisors.
On February 28, 2011, Chiu announced his mayoral candidacy at a morning rally at San Francisco City Hall. Over the course of the campaign, Chiu raised over $1.24 million from both private and public sources and spent roughly the same amount.
On Election Day, Chiu placed fourth behind incumbent Ed Lee with 17,921 first-place votes. Despite the fourth-place finish, Chiu, Lee, and third-place candidate Dennis Herrera appeared individually on more ballots overall than John Avalos, who came in second.
On September 12, 2013, Chiu announced his candidacy for the California State Assembly to replace termed-out Democrat Tom Ammiano in the 17th district, which encompasses the eastern half of San Francisco.
He ran against fellow Democrat and supervisor David Campos. On January 22, 2014, the San Francisco Chronicle column "City Insider" reported that Chiu reported having raised $450,000 for the Assembly race. Polls showed him ahead of Campos.
Chiu beat Campos in the San Francisco primary on Tuesday, June 3, 2014, by approximately five percentage points. Chiu won 48% of the vote, while Campos pulled in 43%, and Republican candidate David Salaverry won 8.5%, meaning the November election would be between Chiu and Campos. On November 4, Chiu defeated Campos with 51.9% of the vote, and Campos conceded on November 6.
David Chiu was appointed by Speaker Toni Atkins to serve as assistant speaker pro tempore in the 2015–16 session. The assistant speaker pro tempore is the third highest ranking position in the state assembly.