The Frenchman sat in the crowd along with fans after injury forced him out of the match, and when one fan and his son asked for a photo, Mendy stunned him as instead he replied "you two can come in with me".
Ben, 27, and his son Ben junior were Mendy's guests of honour in the Etihad's exclusive Tunnel Club as City comfortably defeated Fulham 3-0.
Ben explained: "I don't tend to go in until right before kick-off because it's hard to keep Ben occupied.
"We were walking past the main entrance to get to my seat and I saw Mendy walking in. He's my lad's favourite player because they have the same name so I asked if he'd have a picture taken with us. He just said: 'is it just you two'. I said it was so he told us we were going in with him."
Mendy, 24, handed them both 'access all areas' passes to the £15,000-a-year Tunnel Club.
Ben continued: "I sat behind him and he turned round and told me we'd be sat on his table at half-time and asked us what we'd like to drink. I didn't want to take the mickey so Ben had a Coke and we sat with Mendy and his friends who were all really friendly.
"We went back out [for the second half] and he's picked up Ben and sat him on his knee to watch the second-half. Honestly, he's looked after him and talked to him for about 20 minutes. He didn't even hand him back – Ben junior just came back to sit with me when he wanted to.
"Mendy and his mates said I had to get something to eat with them after the game, so we went back in. They then ordered a bottle of champagne – Mendy wasn't drinking – and made sure I had a glass and Ben was given a goody bag. When it was time to go, I told Ben to go and say 'Merci, Benjamin'. He gave him a cuddle and told him he'd see him tomorrow!
"He was a normal, down-to-earth, friendly guy who did something he really didn't have to.
"He wasn't doing it for publicity, or anything like that, it was just a nice thing to do. I know footballers get a bad press but I cannot give him enough credit – he was absolutely brilliant.
"'It was just a strange sequence of events that led to something like this happening. I never pester players for photos – it was just because it was Mendy and he's Ben's favourite player."