The two were caught in a neck-and-neck presidential nomination race, which was terminated with Obama's win on June 3.
"We may have started on separate paths ... today our hearts are set on the same destination for America ... to elect Barack Obama as the next president of the United States," Clinton told the rally. "We are one party, we are one America."
Unity, which was deliberately selected to be their first appearance location for the symbolism of its name, awarded each of them exactly 107 votes in the New Hampshire primary in January.
Obama also praised the former First Lady for helping to "bring this country a new and better day."
"For 16 months, Senator Clinton and I have shared the stage as rivals for the nomination, but today I could not be happier and more honored and more moved that we're sharing this stage as allies to bring about the fundamental changes that this country so desperately needs," the Illinois Senator said.
"Hillary and I may have started with separate goals in this campaign, but we made history together."
He also directly addressed the tension between them left from the fierce competition for nomination, saying he knew "there have been times over the last 16 months where those biases have emerged," but he also knew "while this campaign has shown us how far we have to go, it has also proven the progress we have made."
The two also attended together a private fundraising event on Thursday night in Washington, D.C., where Obama personally wrote a check of 2,300 U.S. dollars to help Clinton pay her debt of about 22 million dollars left from campaign.
Obama also asked his biggest donors to spare their fortune to help Clinton write off debt.
U.S. media reports earlier said that the two campaigns were still discussing what role Clinton should play for Obama's bid for the White House, and even precisely whether Clinton's name would be placed symbolically into nomination at the party's national convention in late August.
