With Justice Anthony Kennedy's retirement, the Supreme Court will lose its key swing vote and a jurist who earned plaudits from both the left and the right for the weight he put on the rights to privacy, dignity and freedom of speech.

In his more than three decades on the high court, Kennedy was the deciding vote on a number of major cases that helped reshape the U.S. into a country where men and women had the legal right to engage in same-sex sexual activity in every state, where gay and lesbian individuals had the legal right to marriage, where the right of a woman to choose whether to have an abortion remained intact, and where free-speech protections were extended to corporations seeking to make financial expenditures for political campaigns.

"He was the decider — you could not win a case without Justice Kennedy," Tom Goldstein, who argued 41 cases before the Supreme Court before founding the popular court-watching SCOTUSBlog in 2003, told NBC News.

In assessing his 30-year legacy, court watchers and legal experts described Kennedy as a unique jurist who was both dedicated to a core set of conservative values but unafraid to go his own way on key issues.

"While he was a conservative at heart, he would depart from his colleagues on the right of the court when there were issues of privacy and individual rights," said Jonathan Turley, a professor of law at the George Washington University Law School.

Born in 1936, Kennedy graduated from Harvard Law School in 1961 and, after practicing and teaching law in a variety of venues, he was tapped in 1975 by President Gerald Ford for a seat on the 9th U.S. Circuit Court of Appeals. Twelve years later, he was nominated by President Ronald Reagan for the Supreme Court seat that was being vacated by Lewis Powell. Kennedy was Reagan's third choice for the vacancy after his nominations of Robert Bork and Douglas Ginsburg both failed. He was confirmed for the high court in February 1988.

Over the decades, Kennedy, according to legal experts, emerged as a jurist who put a premium on individual rights, particularly in cases that involved social issues, such as abortion, marital rights and privacy.

In 1992, for example, Kennedy sided with four other justices — three liberal justices and Sandra Day O’Connor, a fellow swing vote — in a 5-4 opinion in the Planned Parenthood v. Casey case, effectively upholding the constitutional right to abortion that had been granted 19 years earlier in Roe v. Wade. Kennedy wrote part of the majority opinion in the case, incensing conservatives who had seen it as an opportunity to severely curtail abortion rights.

In 2003, Kennedy joined the court's four liberals — there was also a concurring opinion from O'Connor — in Lawrence v. Texas, striking down a sodomy law in Texas. The decision, effectively, wiped out all state sodomy laws in the U.S. and made same-sex sexual relations legal in every state.

Supreme Court nominee Anthony Kennedy is sworn in before the Senate Judiciary Committee on Capitol Hill in Washington, Dec. 14, 1987. John Duricka / AP file

After O'Connor retired in 2006, Kennedy essentially was left as the only swing vote on the court, giving his voice more prominence in an array of critical legal issues.

In 2013 and 2015, Kennedy wrote the majority opinions in a pair of key gay rights cases:

Siding with the court's four liberal justices, he wrote the 5-4 opinion in U.S. v. Windsor striking down a federal law that denied benefits to same-sex couples.

Legal experts said Kennedy rooted many of opinions in his belief that a "right to dignity" was protected by the Constitution.

"It informed his enormous sensitivity to individual rights dealing with social and marital status," Turley said. "He would speak and write most profoundly and passionately when he was discussing the right to privacy and intimacy and dignity. It's something that resonated clearly and deeply within him."

Turley added that Kennedy also believed that those rulings — regarding the protection of individual rights — were not contrary to a conservative interpretation of the Constitution.

Supreme Court Chief Justice William Rehnquist, right, as he talks with soon-to-be-sworn-in Justice Anthony Kennedy, Kennedy's wife Mary and Kennedy's children, Justin, left, Gregory and Kristin on the steps of the Supreme court on Feb. 18, 1988. James Colburn / AFP-Getty Images file

"The protection of individual rights was part of the mandate of conservatives as much as it was for liberals on the court," Turley said. "That's what makes him a critical bridge figure."

But he was ultimately a conservative, and often the swing vote for the right-leaning wing of the court — especially on freedom-of-speech issues.

In 2010, Kennedy joined with the court's four conservative justices and wrote the 5-4 ruling in Citizens United v. FEC, a controversial case that deemed that the government could not put limits on political campaign spending by corporations.

In his opinion, which has since had enormous ramifications on campaign spending, Kennedy said spending by corporations was a form of political speech, and therefore protected under the First Amendment.

Kennedy "was a reliable vote and frequent opinion writer for traditional views of freedom of speech," said Frederick Schauer, a professor of law at the University of Virginia School of Law.

While experts said they were surprised that Kennedy would decide to leave the court now, knowing that President Donald Trump is likely to nominate someone more conservative, he was probably reassured by the president's pick last year of Neil Gorsuch.

“I think Kennedy probably found that pick reassuring and knows that Trump will make a similar choice this time around,” Turley said.