Sepúlveda is a close ally of Bill de Blasio, being the first assembly member to endorse him in his 2013 and 2017 mayoral runs.

[1][3] In 2010, Sepúlveda made his first run for public office, losing a bid for the New York State Assembly to incumbent Peter M. Rivera in the Democratic primary.

Sepúlveda was the first member of the Assembly to endorse future New York City Mayor Bill de Blasio in the 2013 New York City Mayoral primary elections[5] as well as the first Assembly member to endorse de Blasio in his 2017 reelection campaign.

[7][8] However, in 2018, Sepúlveda supported incumbent state senator Martin Malave Dilan against a primary challenge from Julia Salazar, a community organizer endorsed by the Democratic Socialists of America,[9] and endorsed former Independent Democratic Conference leader Jeffrey D. Klein against challenger Alessandra Biaggi.

[15] Sepúlveda sponsored the New York State Driver's License Access and Privacy Act, also known as the "Green Light" bill.

[17] In support of Kathy Hochul's nomination of Hector LaSalle as chief judge of the New York Court of Appeals, Sepúlveda spoke at a press conference, saying that "I don't care if this fight costs me my political career.