The exit of McCarthy further clears the field for Gillibrand's bid to win election to the Senate seat Gov. David A. Paterson named her to in a surprise decision in January.
McCarthy has been one of Gillibrand's toughest critics because of her record on guns, which won the National Rifle Association's highest rating. Under McCarthy's prodding, Gillibrand shifted some of her positions on key gun-control measures that she once opposed.
Still McCarthy said repeatedly she didn't want to run but would if no one else did. Thursday, she reversed course.
"Rep. Carolyn McCarthy is not going to run for the Senate for personal reasons," said her spokesman Ray Zacarro, who declined to detail the reasons.
A source familiar with the situation said no one called McCarthy to ask her to drop out, and she's not getting out to clear the field for someone else.
The announcement came on the heels of reports that Rep. Carolyn Maloney (D- Manhattan) will soon announce her primary challenge to Gillibrand.
Vice President Joe Biden called Maloney this week, his spokeswoman Elizabeth Alexander said, but she declined to describe the conversation. A source close to Maloney said he didn't ask her not to run.
Biden's call will not affect Maloney's decision on running, the source said.
Rep. Steve Israel (D-Huntington) last month called off a challenge after President Barack Obama asked him to stay in the House. Manhattan Borough President Scott Stringer folded his Senate exploratory committee days later.