O'Connor made history as the first woman to sit on the Supreme Court. She was appointed in 1981 by President Ronald Reagan. She served until 2006, when she stepped down to care for her husband, who had Alzheimer's disease.
During her tenure on the High Court, O'Connor was known as being a swing vote and cast the deciding vote on many cases, including Planned Parenthood v. Casey, the 1992 case that reaffirmed a woman's right to have an abortion.
She also penned the majority opinion upholding affirmative action in 2003 and, in 2004, wrote the opinion that gave detainees in Guantanamo Bay the ability to petition federal courts to review the legality of their detention.
She also sided with conservatives on several cases relating to religious and states rights.
"A daughter of the American Southwest, Sandra Day O'Connor blazed a historic trail as our Nation's first female Justice. She met that challenge with undaunted determination, indisputable ability, and engaging candor. We at the Supreme Court mourn the loss of a beloved colleague, a fiercely independent defender of the rule of law and an eloquent advocate for civics education. And we celebrate her enduring legacy as a true public servant and patriot," Chief Justice John Roberts said in a statement.