The 54-year-old music mogul was at the Peninsula Hotel during the festivities, E! reported, while many of his ex-lovers -- including Mezhgan Hussainy and Terri Seymour -- were there to celebrate with Silverman.
More details from the celebration: It was decked out in blue and white (as the couple is expecting a boy); a muscle-bound male server gave out drinks; and a stunning floral arrangement was composed using white orchids, yellow orchids, blue hydrangeas and white roses.
Next up for Cowell and Silverman is a holiday vacation to Barbados; insiders tell People that wedding bells might not be far behind for the duo.
"Simon doesn't want Lauren to give birth to his son as a single mom," an insider told the magazine. "He is ready to commit."
Simon and Lauren's baby is due in February.