The singer, who now goes by her married name, Lily Cooper, donned an oversized vintage dress (you needed a microscope to find her hiding under there) for the occasion and worked red lips, flicked eyeliner, a full fringe, and a backcombed updo, which looked stunning.
On Monday she took to Twitter to let fans know the news, writing: "Just did rehearsal for tomorrow's ETAM show with @iamMarkRonson, still know the words to Smile, which is good. #mumback."
It is thought Lily hasn't performed by herself since announcing that she was quitting the music scene back in 2009, playing a 'last' performance at Camden's Koko club, before heading off to have two children.
And her two-month-old bubba, Marnie, watched mum from the audience - a little fashionista in the making.
What do you think of Lily's comeback look?