[6] While in the Assembly, Díaz sponsored, co-sponsored and passed legislation addressing health care,[7] public records access,[8][9] minimum wage and overtime pay,[10] environmental protection,[11][12][13] equitable labor standards,[14] insurance fraud,[15] tenants rights,[16] transparency and disclosure in all environmental impact statements,[17] pedestrian safety,[15][18] school bus safety,[19][20][21] protection from tax preparers,[22][23][24] Senior Citizens rights,[25] wider access to the Senior Citizens Rent Increase Exemption (SCRIE) Program,[26] and the regulation and accountability of gas and electric companies.

[36][37] Díaz has legislated on behalf of Brownfield Cleanup and Green Roof Tax Abatement,[38][39] worked to restore the Bronx River which runs through the 85th Assembly District, and opposed environmental racism.

[41] On February 4, 1999, Amadou Diallo, a young African immigrant, was killed by four New York City police officers who fired 41 unanswered rounds at him.

[44][49][50][51][52] In summer 2008, Díaz became a founding member of a progressive civic and political group known as the "Rainbow Rebels", who achieved sudden and widespread popularity throughout the Bronx County of New York.

[53] On August 22, 2008, the Rainbow Rebels made their first official announcement: Díaz joined with two of his Assembly colleagues Carl Heastie and Michael Benjamin, both Democratic African Americans, and with Assemblyman Jeffrey Dinowitz of Riverdale and his powerful Benjamin Franklin Reform Democratic Club, to promote the candidacy of Elizabeth Taylor for a Civil Court judgeship.

[56] When Mayor Michael Bloomberg declared a special election to choose his successor,[56] Díaz was considered the leading candidate for the position.

[59][60] Díaz ended his first summer as borough president by recommending that the New York City Council reject Related Companies' proposal to turn the Kingsbridge Armory into a shopping mall.

In an editorial in the New York Daily News, Díaz wrote that he is "fighting to make sure that this development includes 'living wage' jobs that offer health insurance".