Clinton said that Obama had "hit one out of the ballpark" when he chose Joseph Biden, the Delware senator who was set to speak at the convention later on Wednesday, as his running mate.
Al Jazeera's Rob Reynolds in Denver said the speech showed the Clinton's had gone out of their way to endorse Obama.
Marwan Bishara, Al Jazeera's senior political analyst, said that Clinton could have seen Obama as continuing his political legacy if wasn't for the bitter primary electioon battle with Hillary Clinton, his wife and the New York senator.
Earlier, Obama had been officially selected as the US Democratic party candidate for November's presidential election after delegates at the party's convention approved his primary poll win by acclamation.
The historic endorsement at the Denver convention hall made the Illinois senator the first African-American to receive the nomination of a major US party.
Obama's former rival, Hillary Clinton, formally asked delegates to suspend a roll call vote of states and approve his nomination by acclamation.
"With eyes firmly fixed on the future, in the spirit of unity, with the goal of victory, with faith in our party and our country, let's declare together in one voice right here right now, that Barack Obama is our candidate and he will be our president," she said to roars of approval inside the packed convention hall.
"I move senator Barack Obama of Illinois be selected by the convention by acclamation as the nominee of the Democratic Party," she said, a request accepted by the convention's presiding official, Nancy Pelosi, the speaker of House of Representatives.
Pelosi then announced that Obama had accepted the nomination.