As Americans prepare to fire up barbecues to celebrate US Independence Day on July 4, the protesters started a "Troops Home Fast" that began at midnight.
Actors, including Sean Penn and Susan Sarandon, are to join the protest.
"We've marched, held vigils, lobbied Congress, camped out at Bush's ranch, we've even gone to jail, now it's time to do more," said Sheehan, who emerged as an anti-war figurehead after her 24-year-old son, Casey, was killed in Iraq.
For their "last meal", several dozen protesters clustered around a tablecloth on the pavement outside the White House fence on Monday.
"We withdraw our consent to a government who commits torture. We withdraw our consent to a government who sent our troops to invade a country which represented no threat to our security," Sheehan told the group.
The protest is the latest attempt by the US anti-war movement to grab hold of American public opinion, after numerous marches, vigils and political campaigns. Sheehan and at least three other activists intend to continue their fast.
Dianne Wilson, one of the campaigners, said: "I don't know how long I can fast, but I am making this open-ended."
Others, including Penn, Sarandon, novelist Alice Walker and actor Danny Glover will join a "rolling" fast, a relay in which 2,700 activists pledge to refuse food for at least 24 hours, and then hand over to a comrade.
While 2,526 US soldiers have died since the US-led invasion of Iraq in 2003, according to Pentagon figures, the impact of the deaths has rarely dominated headlines.
Meredith Dearborn, of human rights group Global Exchange, said: "We have done everything we could think of to end this war, we have protested, held marches, vigils, lobbied, written letters to congress.
"Now it is time to bring the pain and suffering of war home."