Gillibrand, at 42, will be the youngest member of the Senate and one of 17 women in the chamber. The second-term congresswoman will assume the seat once held by Kennedy's uncle Robert F. Kennedy as well as by Daniel Patrick Moynihan.
"For many in New York state, this is the first time you've heard my name and you don't know much about me," said Gillibrand (pronounced JILL-ih-brand). "Over the next two years, you will get to know me. And, more importantly, I will get to know you."
Before the governor even took the podium to introduce Gillibrand, anti-gun crusader Rep. Carolyn McCarthy said she would challenge Gillibrand in the Democratic primary next year, or find someone who would. Gillibrand has a 100 percent voting record with the National Rifle Association.
McCarthy, a Long Island Democrat who ran for Congress after her husband was shot to death and her son wounded in the 1993 Long Island Rail Road massacre, said someone with such a record should not be the next senator from New York.
"The majority of New Yorkers believe in trying to reduce gun violence," she said.
Her complaint was echoed by New York Mayor Michael Bloomberg, a Democrat-turned-Republican-turned-independent who has been one of the nation's most vocal gun control advocates. In a statement, the mayor noted his "strong disagreement with one area of her record as a member of Congress: illegal guns."
Bloomberg and McCarthy said Gillibrand co-sponsored legislation to deny information cities and police need to track illegal gun use. The legislation passed in the House but was never considered by the Senate, McCarthy said.
At the news conference, Gillibrand commended McCarthy for her "outstanding leadership in fighting against gun violence and keeping our children safe," and added: "I pledge to work with her on her signature bill for updating background checks to keep guns out of the hands of criminals." Gillibrand said she would support gun control "but also protect our hunters' rights."
The Senate appointment lasts until 2010, when a special election will be held to fill the final two years of Clinton's term.
In picking Gillibrand, the governor passed over a number of better-known and more accomplished politicians, including New York City Rep. Carolyn Maloney and New York Attorney General Andrew Cuomo.
Paterson cited Gillibrand's work ethic, background as a securities lawyer and knowledge of issues important to New York. He stressed that he didn't choose her because she was a woman or from upstate New York.
Several of those passed over didn't attend the news conference, even after the governor summoned New York's congressional delegation to Albany. Cuomo was among the no-shows, even though Gillibrand worked for him as legal counsel when he was housing secretary under President Bill Clinton.
Paterson "has likely engendered the wrath - privately, if not publicly - of a great number of prominent Democratic officials who would have liked the job themselves," said Steven Greenberg, a Siena College pollster.
President Barack Obama issued a statement praising the choice, saying Gillibrand would be a "strong voice for transparency and reform in government."
Clinton, whose nomination for secretary of state was confirmed by the Senate on Wednesday, also offered praise for Gillibrand, who worked on Clinton's 2000 Senate campaign.
"Kirsten is an intelligent and dedicated public servant and a dear friend. I'm pleased that this seat, which has been my great honor to hold ... will be in such capable hands," Clinton said.
Meanwhile, the "personal reasons" that Kennedy cited in dropping out of contention remained a mystery.
A friend of Kennedy's who was involved in the selection process said Kennedy had a "minor issue with a nanny" that the governor's staff found to be irrelevant. The friend, who spoke on condition of anonymity because of the sensitivity of the situation, also said Kennedy had a $615 city tax lien that was settled in 1994 but no other tax problems.
Gillibrand, a married mother of two sons, modeled her own political career after Clinton's.
Her father is one of the most powerful lobbyists in the state capital, and she is the granddaughter of Polly Noonan, an intimate of the legendary Albany Mayor Erastus Corning.
After Gillibrand worked in New York City for a white-shoe law firm with long ties to the Democratic Party, Clinton introduced her to deep-pocketed donors and vouched for her as a candidate.
As a lawmaker, she has projected a down-home image in tune with her rural district, which stretches from Lake Placid past the state capital of Albany. Besides the backing of the NRA, she has the strong support of the New York Farm Bureau, which described her as "a leading advocate for agriculture in Washington." Her Web site's homepage includes a picture of cows.
Gillibrand beat Republican Rep. John Sweeney in 2006 in a nasty campaign and won again last fall against a former state Republican chairman in the GOP district.