On the afternoon of November 23, 2009, the New Jersey Senate Democrats chose Sweeney as State Senate President over the incumbent, former governor Richard Codey. He took office on January 12, 2010. In the absence of the governor and lieutenant governor, Sweeney served as acting governor of New Jersey during the eastern seaboard storm of December 2010. In January 2010, Senate President-elect Sweeney abstained when the New Jersey Senate voted on the question of allowing same-sex couples to marry. Sweeney called his abstention a mistake and said that the issue was a civil rights issue, not a religious issue. In 2012, Senate President Sweeney was one of the prime sponsors of legislation that would legalize same-sex marriage for all New Jersey residents. The bill was approved in both houses of the Legislature, but ultimately vetoed by Governor Chris Christie, who instead favors putting the issue up for public referendum. In 2010, Senator Sweeney helped design and pass thirty bills, known collectively as "Back to Work NJ," that aimed to help create jobs and economic growth in New Jersey. In 2011, Sweeney proposed sweeping reforms to the public employee pension and health benefits systems that he estimates will save taxpayers over $120 billion over a 30-year period. Sweeney also helped craft the state's two-percent property tax cap in order to control rising property taxes. Senator Sweeney was named as a "Politician Who's Ahead of the Curve" by Philadelphia Magazine in 2011 for his continued support of shared services between local government units. Together with Republican Thomas Kean, Jr. and fellow Democrat Raymond Lesniak, Sweeney sponsored Senate Bill S2664, the "Market Competition and Consumer Choice Act" in 2011, which Verizon says will encourage the firm to create additional jobs in the state, freeing the firm from regulations established when it was the landline monopoly requiring it to pay for service outages and to provide Government-access television (GATV) channels for municipalities on cable TV. The New Jersey State League of Municipalities has opposed the provisions on elimination of Public-access television channels and on rules requiring free Internet access for municipal and school facilities. Consumer groups argue that the bill would lead to lower-quality service and increased rates, though Verizon promised in a letter sent to the bill's sponsors that committed the company to keeping rates level for basic service in the first two years following approval into law. Following Governor Christie's use of the line item veto on the state's 2011 budget, Sweeney was quoted by The Star-Ledger as being incensed. Two days later, Sweeney was unapologetic about what The Star-Ledger described as a "tirade" against Christie, saying "[...] I don't apologize for it. The governor was wrong to hurt people," in response to further questions about the earlier reports which quoted him as describing Christie as a "rotten bastard," a "punk," and "Mr. Potter from It's a Wonderful Life." Among the Governor's cuts was funding for tax credits and health care for the working poor, women's health funding, AIDS medication funding, and mental health services. In January 2013, two months after Hurricane Sandy hit New Jersey, Sweeney suggested that Governor Christie "got lucky" because the hurricane had distracted voters from New Jersey's slow economic recovery, an issue that many political observers believed was a potential point of weakness for Christie. He was heavily criticized for his remark, and a spokesman for Christie called it "politics at its worst". During Chris Christie's tenure as Governor, Sweeney was the lead advocate of legislation to raise taxes on millionaires, forcing Christie to veto it on five occasions. After Democrat Phil Murphy became Governor, Democrats backed off the legislation, with Sweeney saying, "This state is taxed out. If you know anything about New Jersey, they're just weary of the taxes."In December 2018, Sweeney led efforts to change the New Jersey constitution so that it entrenched a gerrymandering of New Jersey districts in a way that would grant Democrats a supermajority and keep Republicans in a permanent minority. The constitutional changes had to be approved through a ballot measure. The efforts were condemned by national Democrats, such as former Attorney General Eric Holder, as well as by the Democratic New Jersey Governor Phil Murphy.