The huge Defensive Tackle was the star of the show as the event - part of NFL’s ‘Play 60’ initiative - gave hundreds of local shoolchildren the chance to try their hand at both soccer and American football skills activities ahead of the Bills' International Series clash against the Jacksonville Jaguars at Wembley Stadium this Sunday.
The children were met by Marcell and a number of players from the Bills squad including quarterback Tyrod Taylor, cornerback Ron Brooks and kicker Dan Carpenter, as well our Belgian trio of Jan Vertonghen, Toby Alderweireld and Mousa Dembele.
Marcell's sheer joy at the event was infectious and he told us afterwards just how important he saw the work of our Foundation and other sporting organisations worldwide in the community.
Below: Spurs TV special - Buffalo Bills thrill at Hotspur Way
"I'm constantly in the community, constantly trying to get the kids out," he said. "If you are going to play for 60 minutes, try and play another 60, get your friends around, turn on some music, have a dance, have a blast, have a good time. I played football with some kids after our game (Bills v Bengals) on Sunday.
"The kids here were looking at me thinking 'wow, an American football player' and they were so excited about the Bills coming. They had a great time, they took it all in. Their eyes were so big, excited about what was going on. I remember that fire in me. I had a lot of fun. The kid inside of me had a real blast."
Already a Pro Bowler twice in his NFL career, it's no secret back home in the USA that Marcell had a tough time as a youngster growing up in Birmingham, Alabama. It's all detailed in an NFL special here.
Below: Marcell was presented with Jan Vertonghen's shirt
So what advice would he give to aspiring young sportsmen?
"My advice is to find something you love, really put your all into it and believe in what you want," he said. "Do what you want to do. Don't let people push you in a direction you don't want to go. Be successful in your own right and have a blast doing it. But make sure it's the right thing. Only do what's right and you'll always be fine."
Those are the words which have helped make him one of the most highly-rated defensive players in the NFL.
"I'm here. I'm in London right now. I could be back home in Birmingham (USA) sitting on the couch doing nothing but I wanted something in life and I kept pushing for it. Even when it got to the point I thought it wasn't going ot be there anymore, I hung on in there a little long and miracles happen."