Before entering office, Alesi ran a firm that operates washers and dryers in apartment complexes and colleges.
Alesi began his political career as the Republican Deputy Town Leader for Perinton, New York. In 1977 he made an unsuccessful bid for a seat in the Monroe County Legislature against Louise Slaughter. He won on his second attempt twelve years later (Ms. Slaughter had left to take a seat in the New York State Assembly and later a seat in the U.S. House of Representatives.) In 1992, Alesi was elected to the New York State Assembly, where his colleagues named him President of his class of freshman legislators. He was elected to the New York State Senate in a special election in 1996, and was re-elected every two years until his retirement in 2012.
In January 2008, Alesi illegally entered an unfinished Perinton home, claiming he believed the house was for sale, and broke his leg while climbing a ladder. The front door was locked, but Alesi managed to gain entry through an unlocked back door. He did not have permission from either the builder or the homeowners to enter the house. The owners and builder declined to press charges for trespassing. On January 18, 2011, the same day the statute of limitations for any potential trespassing charges expired, Alesi filed a lawsuit against the owners and the builder alleging an unsafe environment at the home site. After criticism from members of the public and other Republicans, Alesi withdrew the lawsuit.
In 2009, Alesi voted no on same-sex marriage legislation despite being considered the Republican most likely to support the legislation. He is seen on video from the New York Senate floor casting his no vote with his head in his hands, and later admitted that he struggled with his decision to vote against the legislation.
In 2011, Alesi became the first Republican to announce his support for a new same-sex marriage bill. On June 24, 2011, Alesi was one of four Republicans to vote in favor of the Marriage Equality Act, stating: "I swore with my hand on the Bible to uphold the Constitution ... I didn't swear with my hand on the Constitution to uphold the Bible".
On May 9, 2012, Alesi announced that he would not run for re-election to the State Senate. Alesi stated that many factors, including the welfare of the Republican Party, led to his decision. Sen. Alesi indicated that Senate Majority Leader Dean Skelos had been supportive of his re-election. Prior to Sen. Alesi's announcement, Republican Assemblyman Sean Hanna had expressed interest in running a primary challenge against Sen. Alesi. On the same day that Sen. Alesi announced his decision not to run for re-election, the Daily News reported the following:
Sen. James Alesi (R-Rochester) told the Daily News Wednesday night that he believes his vote in favor of the landmark gay marriage legislation would have severely hampered his chances in a GOP primary this year. He said the vote — which passed 33-29 due to the GOP defectors — cost him the support of the leadership of the local Republican and Conservative parties. They are now backing his primary opponent... Believing that his political brand was weakened by the gay marriage vote, Alesi said he decided to step aside out of fear that a bruising primary would eventually cost the GOP his seat — and with it potentially the party's razor-thin Senate majority. His political future was also clouded, however, when he sued a couple after he broke his leg while breaking into their home, which was under construction. He wound up dropping the suit, but the political exposure from an incident in which he was trespassing was anything but favorable.