Kirk, a Kennedy family friend, served on the senator's staff for eight years and is chairman of the JFK Library Foundation. Kennedy's widow, Vicki Kennedy, and son Edward Kennedy Jr. had urged Patrick to pick Kirk and attended the announcement at the Massachusetts Statehouse.
"He (Sen. Kennedy) often said that representing the people of Massachusetts in the Senate of the United States was the highest honor that he could possible imagine, and it is certainly nothing that I imagined, but it will be my highest honor, as well," said Kirk, who added he expected to be sworn Friday.
Patrick's appointment means Kirk will serve in the interim post until voters pick a replacement in a Jan. 19 special election. Kirk said he would not be a candidate in the special election.
This week, lawmakers gave Patrick the appointment power, five years after taking that power away when Republican Mitt Romney was in office. The legislation did not take effect immediately, so Patrick had to sign an emergency letter Thursday to make the appointment right away.
The interim senator allows President Barack Obama to regain a critical 60th U.S. Senate vote to pass a health care overhaul.
"I am pleased that Massachusetts will have its full representation in the United States Senate in the coming months, as important issues such as health care, financial reform and energy will be debated," Obama said in a statement. "Paul Kirk is a distinguished leader, whose long collaboration with Senator Kennedy makes him an excellent, interim choice to carry on his work until the voters make their choice in January."
Caroline Kennedy, the daughter of President John F. Kennedy, said Kirk's "wisdom, kindness and integrity mean the world to the entire Kennedy family."
"Paul's life long commitment to democracy and civic engagement has given life to my father's belief that one person can make a difference, and earned him the friendship and respect of politicians on both sides of the aisle," she said in a statement.
Sen. John Kerry, who attended the announcement, called Kirk a "superb steward for this seat" and said he already has a personal relationship with many of the people he will be working with during his brief time in Kennedy's former office.
"Paul Kirk shares Ted's love of the commonwealth and the country and shares his passion for public service," Kerry said, adding "he is going to hit the ground running."
Kirk, 71, is a Boston attorney who also has been a registered lobbyist for pharmaceutical companies. He said Thursday he would resign from his board positions, including at Hartford Financial Services, known colloquially as "The Hartford." It sells life and property insurance.
Early in his career, he spent eight years on the senator's staff. He was master of ceremonies last month for a memorial service the evening before Kennedy's funeral.
Kennedy died Aug. 25 after a yearlong battle with brain cancer. Kirk has been helping with efforts to raise money for a Senate institute named for him. Organizers have been criticized for accepting donations from the health care industry while Kennedy and the White House pushed for an industry overhaul.
Federal records show Kirk registered as a lobbyist a decade ago. While Kirk would be banned from lobbying for two years after his appointment ends, he would retain Senate floor privileges, the honorific title "senator" and a coveted Capitol Hill parking space for life.
Kirk said that while he had represented "a couple of pharmaceutical firms" as recently as 2002, he no longer works as a lobbyist and has no conflicts of interest.
Kirk graduated from Harvard College and Harvard Law School and served on Kennedy's staff between 1969 and 1977. He ran the Democratic National Committee in the run-up to former Massachusetts Gov. Michael Dukakis' unsuccessful run for president in 1988.
Kirk also co-founded the Commission on Presidential Debates, which has sponsored every presidential and vice presidential debate involving major candidates since 1988.
Kirk retains graybeard status within the party in part because he has never served in political office. That factored into his selection, one aide said, since he posed no threat to the candidates competing in the special election.
The Massachusetts Legislature laid the groundwork Wednesday for the announcement, when, after a contentious debate and a whirl of parliamentary maneuvering, it approved a bill allowing the governor to make an interim appointment.
Patrick has argued the state stood to suffer without full Senate representation before the special election campaign, but some fellow Democrats have joined Republicans in accusing him of a power grab. Patrick said he was untroubled by criticism from Republican lawmakers.
"I'm quite satisfied that I am both within the law and within tradition," Patrick said.