Ancelotti singled out the star after he helped four-time defending Bundesliga champions Bayern to a 3-1 comeback at Opel Arena.
And Ancelotti was delighted with his impact following criticism of his indifferent start to the season, praising the 27-year-old's influential display.
"Muller played well," the Italian told reporters. "I've always said he's a very intelligent player, who can play in any number of number of positions up front.
"Muller changed his position, but I am happy with all the forwards, in the first half, they showed good combinations together.
"It was a difficult game, but it's a good result. It was important to win here, we played against a difficult team and it's always hard to win in Mainz."
Bayern surprisingly fell behind to Jhon Cordoba's fourth-minute opener away from home.
But Robert Lewandowski's brace and a goal from Robben in his 150th Bundesliga appearance inspired Bayern.
"It was a tough game, but we've got a good result out of it, and that was really important for us," Ancelotti continued.
"We did well going forward in the first half, then Mainz upped the pressure in the second half and it got more difficult."