The transition "must be meaningful, it must be peaceful and it must begin now," Obama said at the White House after speaking with the Egyptian president by telephone for 30 minutes.
In Cairo, Mubarak said he would not seek re-election in September, but he rejected protesters' demands that he step down immediately, a halfway concession that did not calm his critics.
Obama's comments suggested that September might not be soon enough. He did say that Mubarak "recognizes that the status quo is not sustainable and that a change must take place."
Obama said the transition should include a broad spectrum of Egypt's political factions.
He said that demonstrators, who have been mostly peaceful, have been "an inspiration to people around the world."