If confirmed, Kagan will replace outgoing Justice John Paul Stevens who is retiring after 35 years on the bench.
The move positions the court to have three female justices for the first time in history. If confirmed, Kagan, who grew up on the Upper West Side and attended Hunter College High School, would join Bronx-born Justice Sonia Sotomayor on the bench.
Announcing his choice this morning in the East Room of the White House, the president joked that his nominees were ideologically balanced in a way beyond law: the Bronx native Sotomayor is a Yankees fan while Kagan roots for the Mets.
As the Obama administration's top Supreme Court lawyer, Kagan has argued six cases before the people who will become her colleagues if she wins Senate confirmation.
In making the announcement, President Obama described her as a "friend" and one of the "foremost legal minds" in the United States. He praised her "openness to a broad array of viewpoints," saying she makes a point of "understanding before disagreeing."
For her part, Kagan said she was "humbled and honored" by the nomination and praised the Supreme Court as "an extraordinary institution in the work it can do for the American people by advancing the tenets of our Constitution, by upholding the rule of law and by enabling all Americans, regardless of their background or their beliefs, to get a fair hearing and an equal chance at justice."
Kagan thanked her family for their support throughout her legal career. Her two brothers are both New York City school teachers, the kind of educators "whom students remember for the rest of their lives."
Known as sharp and politically savvy, Kagan has blazed a remarkable legal career: first female dean of Harvard Law School, first woman to serve as the top Supreme Court lawyer for any administration, and now first in Obama's mind to succeed the legendary Justice Stevens.
Even in her high school years on the Upper East Side, Kagan showed her belief in the power of government.
The quotation she selected to run with her photo in the Hunter College high school yearbook in 1977 was from former Supreme Court Justice Felix Frankfurter. It reads: "Government is itself an art, one of the subtlest of arts,"
Kagan also showed a lighter side by selecting a quotation from Mark Twain's "Huckleberry Finn:" ''Persons attempting to find a motive in this narrative will be prosecuted; persons attempting to find a moral in it will be banished; persons attempting to find a plot in it will be shot."
At 50 years old, Kagan would be the youngest justice on the court, one of many factors working in her favor. She has the chance to extend Obama's legacy for a generation.
Kagan has clerked for Thurgood Marshall, worked for Bill Clinton and earned a stellar reputation as a student, teacher and manager of the elite academic world. Her standing has risen in Obama's eyes as his government's lawyer before the high court over the last year.
Yet Kagan would be the first justice without judicial experience in almost 40 years. The last two were William H. Rehnquist and Lewis F. Powell Jr., both of whom joined the court in 1972. All of the three other finalists Kagan beat out for the job are federal appeals court judges, and all nine of the current justices served on the federal bench before being elevated.
A source close to the selection process said a central element in Obama's choice was Kagan's reputation for bringing together people of competing views and earning their respect.
Kagan came to the fore as a candidate who had worked closely with all three branches of government, a legal mind with both a sense of modesty and sense of humor.
Kagan's fate will be up to a Senate dominated by Democrats, who with 59 votes have more than enough to confirm her, even though they are one shy of being able to halt any Republican stalling effort.
In a statement issued before Kagan had completed her remarks, the lawmaker who will preside over her confirmation hearing,Democratic Sen. Patrick Leahy of Vermont, said, "The Senate should confirm Ms. Kagan before" Labor Day.
"Our constituents deserve a civil and thoughtful debate on this nomination, followed by an up-or-down vote, he said.
The Republican leader, Sen. Mitch McConnell of Kentucky, said his party would make sure there was a ``thorough process, not a rush to judgment" on the nomination.
For the second straight summer, the nation can expected an intense Supreme Court confirmation debate even though, barring a surprise, Kagan is likely to emerge as a justice. Supreme Court justices wield enormous power over the daily life of Americans. Any one of them can cast the deciding vote on matters of life and death, individual freedoms and government power.
Presidents serve four-year terms; justices have tenure for life.
Republicans have shown no signs in advance that they would try to prevent a vote on Kagan, but they are certain to grill her in confirmation hearings over her experience, her thin record of legal writings and her objections to the military's policy about gays.