The Hollywood star helped sing the chorus of Neil's hit Sweet Caroline while excited guests cheered and captured the comedy moment on their camera phones.
The Descendants actor was the guest of honour at the gala – where he was presented with an award for his humanitarian work.
Actress Shirley MacLaine – currently appearing in Downton Abbey – gave him the Brass Ring Award, hailing him as the "most generous, sweetest, charming human being" she's ever known.
Originally on presenting duty was George's Ocean's Thirteen co-star Carl Reiner, but he was unable to attend due to sickness.
Shirley told the crowd that the actor was "all right but not coming," while George praised the star as "everything I want to be in my life, except 90".
George was supported at the event by his glam girlfriend Stacy Keibler, who – as usual – was on gorgeous form.
The former WWF wrestler sparkled in a low cut, black and red dress adorned with sequins.
The Hollywood couple were joined by a plethora of stars at the event – which raised $2.5 million for the Barbara Davis Center for Childhood Diabetes.
Among them were former Desperate Housewife Nicollette Sheridan, Dancing with the Stars dancer Julianne Hough, Carmen Electra, Mimi Rogers and Nicki Hilton.