Buono ran against and beat incumbent Republican State Assemblywoman Joanna Gregory-Scocchi in 1994, who had been chosen by a Republican special convention in February 1994 to fill the vacant seat of Republican Assemblywoman Harriet Derman (who was chosen by Governor Christine Whitman to head the New Jersey Department of Community Affairs). In the November 8, 1994, special election, early favorite Gregory-Scocchi was defeated by Buono, after disclosures that a temporary employment firm owned by Gregory-Scocchi had hired illegal immigrants, with Buono having received 27,229 votes (53.74%) and Gregory-Scocchi 23,436 votes (46.26%). Buono was sworn into office on December 1, 1994. After serving seven years in the New Jersey General Assembly she was elected to the New Jersey Senate on November 6, 2001. Buono served in the Senate on the Budget and Appropriations Committee (as Chair), the Intergovernmental Relations Commission and the Joint Budget Oversight Committee. She was the first woman to serve as Chair of the Senate Budget and Appropriations Committee. At the beginning of the 2010 session, Senator Buono became the first woman to be elected Majority Leader of the New Jersey State Senate. She held the position through the end of the 2011 session, when she was succeeded by Loretta Weinberg, after Buono declined to agree with Senate President Stephen Sweeney on the terms of a power-sharing deal.