In his various writings and books, he has addressed animal rights, feminism, drug prohibition, same-sex marriage, Keynesian economics, law and literature, and academic moral philosophy, among other subjects.

Posner has generally been identified as being politically conservative; in recent years, however, he has distanced himself from the positions of the Republican Party,[9] authoring more liberal rulings involving same-sex marriage and abortion.

[12][13] After high school, Posner studied English literature at Yale University, graduating in 1959 with a B.A., summa cum laude, and membership in Phi Beta Kappa.

[16] On October 27, 1981, Posner was nominated by President Ronald Reagan to a seat on the United States Court of Appeals for the Seventh Circuit vacated by Judge Philip Willis Tone.

[19] He is also well known for writing on a wide variety of current events including the 2000 presidential election recount controversy, Bill Clinton's affair with Monica Lewinsky[17] and his resulting impeachment procedure,[20] and the 2003 invasion of Iraq.

[citation needed] Posner's greatest influence is through his writings on law and economics; The New York Times called him "one of the most important antitrust scholars of the past half-century."

Robert S. Boynton wrote in The Washington Post that he believed Posner would never sit on the Supreme Court because despite his "obvious brilliance," he would be criticized for his occasionally "outrageous conclusions," such as his contention "that the rule of law is an accidental and dispensable element of legal ideology," his argument that buying and selling children on the free market would lead to better outcomes than the present situation, government-regulated adoption, and his support for the legalization of marijuana and LSD.

As a judge, with the exception of his rulings with respect to the sentencing guidelines and the recording of police actions, Posner's judicial votes have always placed him on the moderate-to-liberal wing of the Republican Party, where he has become more isolated over time.

[30] In June 2016, Posner was criticized by right-wing media organizations for a column he wrote for Slate in which he stated, "I see absolutely no value to a judge of spending decades, years, months, weeks, day, hours, minutes, or seconds studying the Constitution, the history of its enactment, its amendments, and its implementation.

In a 2000 Yale Law Journal book review on the title "Rattling the Cage" by Steven M. Wise, Posner again criticized the legal notion of animal rights.

In the review, Posner argues that Wise's approach, using the cognitive ability of animals compared to that of very young normal human beings as a basis for rights-worthiness, is arbitrary and in contrast with major traditional and contemporary philosophies (including the theology of Thomas Aquinas for one and utilitarianism for another).

He further states that people whose opinions were changed by consideration of the philosophical arguments presented in Singer's book Animal Liberation failed to see the "radicalism of the ethical vision that powers [their] view on animals, an ethical vision that finds greater value in a healthy pig than in a profoundly retarded child, that commands inflicting a lesser pain on a human being to avert a greater pain to a dog, and that, provided only that a chimpanzee has 1 percent of the mental ability of a normal human being, would require the sacrifice of the human being to save 101 chimpanzees.

Posner is "one of the founding fathers of Bluebook abolitionism, having advocated it for almost twenty-five years, ever since his 1986 University of Chicago Law Review article[40] on the subject.

"[41] In a 2011 Yale Law Journal article, he wrote: The Bluebook: A Uniform System of Citation exemplifies hypertrophy in the anthropological sense.

"[43] At the Cybercrime 2020: The Future of Online Crime and Investigations conference held at Georgetown University Law Center on November 20, 2014, Posner, in addition to further reinforcing his views on privacy being over-rated, stated that "If the NSA wants to vacuum all the trillions of bits of information that are crawling through the electronic worldwide networks, I think that's fine.

"Privacy is mainly about trying to improve your social and business opportunities by concealing the sorts of bad activities that would cause other people not to want to deal with you."

[46][47][48] His co-blogger Gary Becker simultaneously posted a contrasting opinion that while the Internet might hurt newspapers, it will not harm the vitality of the press, but rather embolden it.

In September 2014, Posner authored the opinions in the consolidated cases of Wolf v. Walker and Baskin v. Bogan challenging Wisconsin and Indiana's state level same-sex marriage bans.

"[54] In 2007, Posner wrote the majority opinion upholding Indiana's photo identification law in the Crawford v. Marion County Election Board case.

He stated that judges "weren’t given the information that would enable that balance to be struck" between preventing fraud and protecting voters’ rights.

[2] In 2021, using a modified methodology (including the HeinOnline database and searches for citations to books), Shapiro found that Posner was the most-cited United States legal scholar, generating 48,852 cites to runner-up Cass Sunstein's 35,584.

[3] In his decision in the 1997 case State Oil Co. v. Khan, Posner wrote that a ruling 1968 antitrust precedent set by the Supreme Court was "moth-eaten", "wobbly", and "unsound".

[64] In 1999, Posner applied the lex loci delicti commissi rule on choice of law rather than the Restatement of Torts, Second when rejecting a claim by an Illinois dentist who slipped and fell in Acapulco, Mexico.

[66] In 2003, Posner found that co-workers who did not prevent a hypoglycemic diabetic's fatal attempt to drive himself home violated no duty to rescue.

[67] In Morin Building Products Co. v. Baystone Construction, Inc. (1983), Posner held that the Uniform Commercial Code presumes contracts impose an objective standard upon what would subjectively be illusory promises.

[68] In 1987, Posner dissented when Judge Frank H. Easterbrook, joined by Richard Dickson Cudahy, found that a stockbroker could sue his former employer under SEC Rule 10b-5 after he quit shortly before the firm's lucrative unannounced merger.

[69][70] In 1990, Posner found that Delaware corporate law did not permit an airline's board from adopting a poison pill provision that encouraged its machinists to take strike action if its pilots' takeover attempt succeeded.

[72] In 1984, Posner wrote for the en banc circuit when it held that a consent decree regulating law enforcement Red Squads did not apply to FBI terrorism investigations, over the dissent of Judge Richard Dickson Cudahy.

[73] In United States v. Marshall (1990), Posner dissented when Frank H. Easterbrook, writing for the en banc circuit, held that the punishment for possession of LSD is determined by the weight of the carrier it is found within.

[76] In 2000, Posner found that partners at a big law firm could be considered employees with regard to the Age Discrimination in Employment Act of 1967.