With the stroke of President Obama’s pen, his health care overhaul — the most sweeping social legislation enacted in decades — became law on Tuesday.

Mr. Obama affixed his curlicue signature, almost letter by letter, to the measure, the Affordable Health Care for America Act, during a ceremony in the East Room of the White House, surrounded by House Speaker Nancy Pelosi, Senate Democratic Leader Harry Reid and a raft of other lawmakers who spent the past year on a legislative roller-coaster ride trying to pass it. Aides said he would pass out the 20 pens he used as mementoes.

The ceremony included two special guests: Vicki Kennedy, the widow of the late Senator Edward M. Kennedy of Massachusetts, who had been a driving force for health care legislation before his death last year, and Connie Anderson, the sister of Natoma Canfield, the Ohio cancer survivor whose struggle to pay skyrocketing health insurance premiums became a touchstone of Mr. Obama’s campaign to overhaul the system.

Mr. Kennedy’s son, Representative Patrick Kennedy of Rhode Island, was also there, carrying a gift for the president: a copy of a bill his father introduced in 1970 to provide national health insurance. On it, the younger Mr. Kennedy had written a personal message to Mr. Obama.