The mediator, South African President Thabo Mbeki, introduced Mugabe as president, main opposition leader Morgan Tsvangirai as prime minister and Arthur Mutambara, leader of a smaller opposition faction, as deputy prime minister moments before the three signed the document.
The deal resulted from three months of difficult negotiations mediated by Mbeki at the behest of the Southern African Development Community.
Leaders of the 14-member regional bloc and of the African Union were at Monday's ceremony.