Mukasey attended Columbia University (B.A., 1963) and Yale Law School (J.D., 1967). After working in private practice from 1967 to 1972, he served as an assistant U.S. attorney in New York City. Returning to private practice in 1976, he joined the firm of Patterson, Belknap, Webb & Tyler, where he represented such high-profile clients as lawyer Roy Cohn, socialite Claus von Bulow, the New York Daily News, and The Wall Street Journal. In 1987 Mukasey was nominated by Pres. Ronald Reagan (Reagan, Ronald W.) for a federal judgeship in the Southern District of New York. He rose to chief judge in 2000 and retired from the bench in 2006. From 1993 Mukasey taught law at Columbia University.

As a federal judge, Mukasey was viewed as fair-minded. His views on the role of law and the courts in national security were believed to have been shaped by his experience in the federal courtroom. He oversaw the trial of Sheikh Omar Abel Rahman, who was sentenced to life in prison for planning to bomb the United Nations and other sites in New York City. Mukasey received death threats during that trial. He also presided over the trial of Jose Padilla, a U.S. citizen accused of being a terrorist. After the Sept. 11, 2001, attacks (September 11 attacks) on the United States, Mukasey drew criticism for approving the detention of Muslim men as material witnesses in terrorism cases.

Mukasey supported the administration of Pres. George W. Bush (Bush, George W.) in its attempts to expand executive powers in the name of national security. He was especially outspoken in his approval of the USA Patriot Act, which increased the government's power to detain noncitizens, conduct surveillance and searches, and investigate persons suspected of involvement in criminal activity. Mukasey had no previous ties to the Bush administration, but this stance was seen as a crucial element in Bush's decision to nominate him to the position of attorney general in 2007. It also led to complications during Senate confirmation hearings. In those hearings Mukasey refused to define waterboarding—a type of simulated drowning used to terrify detainees during interrogations—as torture. He also said that he believed that the Constitution gave the president the power to override federal law in some cases. These statements triggered criticism and opposition from many Democrats, and a nomination process originally thought to be a formality suddenly became contentious. The Senate ultimately voted 53–40 to approve the nomination.

On Nov. 9, 2007, Mukasey was sworn in as the 81st U.S. attorney general. Mukasey was the third attorney general to serve under Bush. He replaced Alberto Gonzales (Gonzales, Alberto R.), who resigned after becoming embroiled in a number of controversies, which included accusations that he helped the administration legally justify the torturing of detainees suspected of terrorism.