The music mogul turns 50 on Wednesday (07Oct09) but he got the party started early on Saturday (03Oct09), inviting 400 guests to the event at billionaire Sir Philip Green's mansion in Hertfordshire, England.
And the birthday boy spared no expense in entertaining family, friends and A-list celebrities such as Sir Elton John and Kevin Spacey - he is said to have paid a whopping $1.5 million (£1 million) for the party, including $75,000 (£50,000) for a fleet of limousines to shuttle guests to the venue.
He also treated revellers to vintage champagne and performances by boyband Westlife as well as The X Factor winners Lewis and Alexandra Burke, and a rendition of Happy Birthday from supermodel Kate Moss.
Giant images of Cowell were plastered all around the party, while waiters wore masks bearing his likeness. Pre-recorded birthday wishes from David and Victoria Beckham and Jennifer Aniston were played on giant screens.
Cowell's guest list included his American Idol colleagues Ryan Seacrest and Randy Jackson, as well as Naomi Campbell, Sharon Osbourne, Olivia Newton-John, Dannii Minogue and his 82-year-old mum Julie.
Speaking about the party's hefty price tag, the birthday boy said, "I always pick up my own tab whether it’s for four people or 400."