Holder told reporters Tuesday that Clement is "doing that which lawyers do" to take on the responsibility of representing the lawmakers who wrote the ban.
"Paul Clement's a great lawyer," Holder said. "He has done a lot of really great things for this nation."
President Obama ordered Holder's Justice Department in February to stop defending the 1996 Defense of Marriage Act, which defines marriage as between a man and a woman. That led House Republicans to hire Clement to take up the cause.
But Clement's firm, Atlanta-based King & Spalding, announced Monday that it was withdrawing from the case amid criticism from gay rights advocacy groups. Clement is moving to another firm to continue the work.
Holder's comments came in response to a question during a half-hour news conference in the conference room of his Justice Department office.
The attorney general also said he saw some "disturbing" things in the oil and gas markets that will be examined as part of a task force announced last week to investigate whether fraud and manipulation are behind the recent surge in gasoline and other energy prices.
Asked if he believed there was any reason to suspect criminal activity may be at work besides market forces, he said: "There are at least a couple of things that are, I think, disturbing." He declined to give specifics.
"But again I don't want go too far out there and say that that is the reason why we have seen these gas price rises, but there is, I think, a basis for us to announce the formation of the task force," he said.
The attorney general also was asked why no senior Wall Street executives have been charged for the financial crisis and whether he suspected crimes were committed. He said the department is looking at actions by some individuals, but "whether that will result in prosecutions, I don't know."