The US Senate has confirmed Elena Kagan's Supreme Court nomination, making her the third woman on the nine-member court and the fourth in history.
Five Republicans supported President Barack Obama's pick, with Nebraskan Ben Nelson the only Democrat to oppose it.
Mr Obama welcomed her confirmation, but opponents said she would advocate for liberal policies and criticised her lack of judicial experience.
Ms Kagan is unlikely to alter the court's ideological make-up.
Her swearing-in will mark the first time three women have sat on the court at the same time.
"I am confident that Elena Kagan will make an outstanding Supreme Court justice," Mr Obama said in Chicago. "And I'm proud of the history we're making with her appointment."
He praised her "formidable intelligence, her rich understanding of our Constitution, her respect for the rule of law and her excellent and occasionally irreverent sense of humour".
He also said that Ms Kagan understood that law "isn't just an abstraction or an intellectual exercise".
Ms Kagan was a White House aide to President Bill Clinton and later dean of Harvard Law School.
When he took office, Mr Obama named her US solicitor general, the lawyer who argues the government position before the Supreme Court.
He nominated her to the nation's highest court in May, when Justice John Paul Stevens announced he was retiring.
Ms Kagan, a native of New York, is unlikely to alter the court's narrowly divided ideological make-up. She is Mr Obama's second nomination, after Justice Sonia Sotomayor, the court's first Hispanic justice, and also a New Yorker.
Thirty-six of 41 Senate Republicans opposed her nomination, arguing that her lack of judicial experience and what they described as her liberal political views made her unsuited for a high court that is meant to be non-partisan and apolitical.
Ms Kagan worked as a clerk for Justice Thurgood Marshall, the first black Supreme Court justice and a hero to liberals.
In addition, during her tenure at Harvard, Ms Kagan barred US military recruiters from using school facilities over concerns about the military's policy banning openly gay people from serving.
"The massive expansion of government power we've seen over the last 18 months puts our country at a unique time in history," Senator Kit Bond, a Missouri Republican, said in a statement.
"Putting a check on activist government power and adhering to our Constitution is too important to take a chance on a nominee with a lack of judicial experience and history of liberal advocacy."
The last US Supreme Court justice who had not previously been a judge was Chief Justice William Rehnquist, one of the court's most conservative justices. President Richard Nixon nominated him to the Supreme Court in 1971.
Among the most high-profile cases working their way though the legal system, on which Ms Kagan may eventually have to rule, are a lawsuit challenging the constitutionality of Mr Obama's healthcare reform programme and one arguing that the US Constitution requires states to allow same-sex marriage.