The Australian singer was one of many celebrities who attended the star's surprise festivities, which were organised by his close friend Jackie St Claire.
It is believed that St Claire decided to share her own birthday party with Cowell after hearing that he would be too busy to celebrate his 50th when he reaches the milestone in October.
According to the Sunday Mirror, Minogue's performance, which featured Louis Walsh and other guests on backing vocals, provided one of Cowell's favourite moments of the night.
A source revealed: "Jackie went to extraordinary lengths to organise the bash for Simon. She even told the guys performing they were singing for her - but it turned out they were there for Simon.
"She knew the performance would really make him laugh. Dannii took the lead vocals and the others came out providing the comedy for the show. Jackie knew Simon would find it hysterically funny - especially when Louis joined in the singing."
The insider added: "Simon is so vain but, unlike some, he is the first to admit it. So it was the perfect joke."
Amanda Holden, Randy Jackson and Myleene Klass were also in attendance at the party.