A Manhattan Criminal Court judge sentenced them immediately to conditional discharge, which means they could face some form of penalty if they are arrested in the next six months, and ordered them to pay $95 in court surcharges.
Sheehan and about 100 other members of a group called Global Exchange were rebuffed last March when they attempted to take a petition with some 72,000 signatures to the U.S. Mission's headquarters across a street from the United Nations.
Prosecutors said they were arrested after ignoring police orders to disperse.
The four were acquitted of disorderly conduct, resisting arrest and obstructing government administration. They had faced up to a year in jail if convicted of all counts.
"We should never have been on trial in the first place," Sheehan said in a statement after the verdict. "It's George Bush and his cronies who should be on trial, not peaceful women trying to stop this devastating war. This verdict, however, will not stop us from continuing to work tirelessly to bring our troops home."
Sheehan, 49, of Vacaville, Calif., lost her 24-year-old son Casey in Iraq on April 4, 2004. She has since emerged as one of the most vocal and high-profile opponents of the war, drawing international attention when she camped outside President Bush's Texas ranch to protest the war.
The women, calling their campaign "Women Say No To War," had hoped to give the petition to Peggy Kerry, the mission's liaison for non-governmental organizations and sister of Sen. John Kerry, D-Mass., as they had in 2005.
Kerry refused to meet with the women in the presence of Sheehan and the news media. She testified during the trial that the presentation seemed like a publicity stunt.
The women ignored police orders to leave and were reading it aloud on the sidewalk when officers arrested them. The women sat on the sidewalk and were carried to patrol wagons.
After Monday's court session, the women returned to the U.S. Mission to ask for an apology.
Richard A. Grenell, the mission's director of external affairs, met them. "When I said I would accept the petitions and asked for them, they said they didn't have the petitions with them," he said.
Sheehan's co-defendants were Melissa Beattie, 57, of New York; Susan "Medea" Benjamin, 54, of San Francisco; and Patricia Ackerman, 48, of Nyack, N.Y.