The 43-year-old pop beauty had a star-studded audience at London's O2 Shepherd's Bush Empire with the 44-year-old beauty - better known as Ginger Spice in the girl group - seated with her close pal Sally Wood - Rolling Stones rocker Ronnie Wood's wife - upstairs at the venue.
Aside from performing most of her solo LP 'Version of Me', Mel teased fans by saying she would be doing ''something spicy'' later on.
The brunette babe - who was Sporty Spice in the 'Viva Forever' group - dedicated the Spice Girls' 1996 hit 'Say You'll Be There' to Geri.
Prior to Melanie coming on, Geri was greeted by chanting fans, who shouted: ''Geri, Geri, Geri!''
She happily stood up and took her own video of the crowd.
Ahead of Melanie's set, Geri shared a photo with her on Instagram, writing: ''Go girl Saturday night out (sic).''
She also shared a short clip of herself singing along, adding: ''Amazing! Really loved the show! (sic)''
Geri supporting Melanie comes after she and Victoria Beckham ruled themselves out of a reunion of the 90s girl group.
Mel followed in Victoria's footsteps by turning down the chance to join forces with Geri, Emma Bunton and Mel B for the 'Wannabe' hitmakers' 20th anniversary comeback, and said it was because of seven-year-old daughter Scarlet.
She previously said: ''I had to think about myself and the most important thing in my life, which is being a mother to my little girl. I had to be present for her.
''It was a hard decision, not only letting down the girls but all the fans who want it to happen.
''But we're four individuals, we've gone on to have separate lives and careers and want different things.
''We always said we had to be comfortable but lots of people were getting involved and I wasn't comfortable.''