After the Happy Land Social Club fire claimed the lives of 87 people in 1990, Kavanagh helped coordinate the city's response to the tragedy on behalf of Mayor David Dinkins, co-designing a task force that shut down the most grievous fire code offenders.

[7] At the Mayor's Office, Brian also played a key role in launching the New York City Department of Homeless Services and he then served as the agency's first Policy Director.

[8] As Chief-of-Staff for then-New York City Council member Gale Brewer, Kavanagh negotiated enactment of the Domestic Worker Protection Act, promoting the rights of housekeepers and caregivers.

[18] In 2017, Kavanagh announced that he would run in the special election to succeed Senator Daniel Squadron, who was resigning to found a non-profit.

[19] Not without criticism, Kavanagh was nominated as the Democratic candidate, despite representing very little of the same territory in the state Assembly that the Senate district encompassed.

[23] With Democrats taking the majority in the Senate in 2019, Kavanagh was named Chair of the Committee on Housing, Construction and Community Development.

[24] In his first year as Chair, Kavanagh led the effort to pass the Housing Stability and Tenant Protection Act of 2019 (HSTPA), which became law.

Kavanagh was a co-sponsor of the Climate Leadership and Community Protection Act (CLCPA),[25] enacted in 2019, and has served on the Environmental Conservation Committee in each of his years in the legislature.

[30] Kavanagh is one of six children of an Irish-immigrant police officer and a community leader in Staten Island who worked at a local newspaper.