In November 2019, Preston won a special election to finish Mayor London Breed's term on the Board of Supervisors.

He was re-elected in 2020 but was defeated in 2024 by Bilal Mahmood Born and having grown up in New York City, Preston graduated from Bowdoin College with a degree in anthropology and economics before moving to San Francisco.

[15] In July 2018, Preston, a member of the San Francisco chapter of the Democratic Socialists of America (DSA),[16][17] became a candidate for the 2019 District 5 Supervisor election, to fill the vacancy left when Breed became mayor.

[23] Preston authored San Francisco's 2018 Proposition F, which directs the city to establish a universal right to counsel for tenants facing eviction.

[25][26] After joining the Board of Supervisors, Preston continued to advocate for the program that provides legal representation to tenants in eviction court for the duration of their cases.

[25] In April 2020, Preston introduced an ordinance to permanently bar eviction of tenants for failure to pay rent because of issues related to the COVID-19 pandemic.

[28] The measure, which passed the Board of Supervisors in June 2020 by a vote of 10 to 1, not only barred evictions but also prohibited fees, penalties, interest and other charges incurred due to the pandemic.

[36] In September 2021, the Board unanimously adopted emergency legislation introduced by Preston to temporarily bar "no-fault evictions due to owner move-ins, condo conversions, breach of contract, capital improvements, renovations and demolition of a unit".

[37] In October 2021, Preston started a tenant outreach campaign promoting his district as an “eviction-free zone", including programs to educate residents about ways to avoid eviction and obtain rental assistance.

[40] In April 2020, Preston co-introduced legislation with Supervisors Matt Haney, Hillary Ronen and Shamann Walton to require Mayor Breed to secure 8,250 hotel rooms to house the homeless during the COVID-19 pandemic.

[41][42] Preston also raised the funds to rent 30 rooms at the Oasis Inn near City Hall to house homeless people.

[43] The following month, he supported the establishment of "safe sleeping sites", including one in his district at an empty lot in the Haight-Ashbury neighborhood.

[59] In 2020, Preston delayed a major zoning plan which would have led to the construction of thousands of housing units to the Van Ness/Market Street area of San Francisco.

Proposition I raised the transfer tax rate for property sales valued over $10 million, intended to fund affordable housing.

[72] In October 2021, Preston voted against the construction of a 495-unit apartment complex (one-quarter of which were designated as affordable housing) on a parking lot next to a BART station.

Preston’s legislation creates a working group to generate a business and governance plan for a public bank to be presented to the Board of Supervisors.

[77] A few months after taking office, Preston introduced a resolution opposing proposed fare increases by the San Francisco Municipal Transportation Agency (SFMTA).

Preston's resolution introduced in February 2020 called on the SFMTA to avoid fare increases throughout the fiscal year 2021–2022 budget cycle.

[79] In response, Preston and Supervisor Aaron Peskin proposed a charter amendment to strip the authority to increase fares from the SFMTA board.

[84] In December 2021, Preston also proposed a resolution which passed unanimously calling an end to street parking at about 1,000 of the city’s bus stops.

[88] After significant backlash from residents, the Board reversed its decision a week later by unanimously approving an amended policy barring SFPD from using robots to kill.

[89][90] Preston introduced ballot measure Proposition H in May 2022 to shift local elections from odd-numbered to even-numbered years in an effort to increase voter participation.