“It’s extra special for me actually because last April I suffered a minor stroke. And I thought I wouldn’t get back on stage and I have done it.”
The Britain’s Got Talent appearance is believed to be Boyle’s first performance since her stroke.
Judge Simon Cowell thanked her for returning to the talent show stage, saying it owed a lot to her.
He revealed that he had been aware of her health battle and was impressed by her performance on the night.
The Scottish star’s appearance was a closely guarded secret until she took to the stage, joining the cast of West End musical Les Miserables for a live performance, duetting with X Factor star Lucie Jones on I Dreamed a Dream - the song that first made her famous.
Boyle was spotted in August last year, four months after her stroke, in a rare outing when she stopped to pose for photos with fans at the Knock House Hotel in County Mayo.
The star smiled for a photo with the hotel owner, who revealed she was a “fabulous lady”.
The owners later wrote on their Facebook page, “It was our pleasure to welcome Susan Boyle back to Knock House again recently. Susan is a fabulous lady and we always love to see her return time and time again to Knock House Hotel.”
It was the first time the star had been snapped in public in over a year.