Alfie Baugh, aged 8, from Tettenhall, organised and ran the session at Waitrose on Penn Road, Wolverhampton.
Hundreds of visitors piled through the doors of the supermarket to sample a range of cakes and hot drinks at the event, which was opened by the the Mayor of Wolverhampton, Councillor Ian Brookfield.
Alfie, who was named one of the city's Young Citizen's of the Year by Wolverhampton's Rotary clubs in July, said he organised the coffee afternoon in honour of his grandad, Albert Leary, who died from cancer five years ago.
"I'd seen adverts for Macmillan on television and thought it would be a nice way to raise some money in my grandad's memory," said Alfie, who raised more than £600 for charity earlier this year by selling cakes to his neighbours.
"It took a lot of organising. I ordered all the kit and did a leaflet drop around Tettenhall. I'd like to thank everyone who has helped me to plan out the day."
Food and drinks were provided by Waitrose, Greggs and Baked in Tettenhall, while extra staff were brought in to serve customers in the supermarket's cafe.
The city's mayor was at the head of the queue - fresh from attending five Macmillan events over the course of 24 hours.
"It's been a remarkable 24 hours, but I've definitely saved the best until last," he said.
"Alfie is a remarkable young man and his determination to bring everyone together to raise funds for a worthy cause is to be commended."
Waitrose got on board with the event after Alfie pitched his idea to staff at the store.
Waitrose manager Amanda Vaughan, said: "Alfie has organised this from start to finish. We're just happy to be able to support him."
Alfie was hoping to raise more than £1,000 from the event.