LONDON—Carla Bruni-Sarkozy has shared the world stage with Britain’s queen, the pope and the Dalai Lama.

But in London on Tuesday night, France’s first lady appeared on a different kind of stage, this time with rock royalty—the Kings of Leon and Metallica.

Introduced simply as Carla Bruni, she sang live on the British Broadcasting Corp. television program “Later Live ... With Jools Holland” and was interviewed by the pianist.

Sitting at a grand piano, Bruni-Sarkozy was asked how she managed to balance her work as a musician with her duties as France’s first lady.

“I don’t really mix them up. I separate them. I play my music and then I go with my husband when he needs me,” the former model said. “He’s not really a critic, or a fan, but he’s involved in my music.”