Photographer killed by runaway bike at British Motocross Championships
A rider fell off his bike during a jump, and the machine crashed through a barrier of bales, leading to the accident.
Mitchell was in his 50s and "taking pictures in a place he adored" in Foxhill, Swindon, when he died.
Another man, a marshal at the event, suffered serious injury in the same incident.
Nikki Dancey, Mitchell's sister, said his family were "devastated".
"I think the fact that he was where he was, doing what he loved, I think in time that will bring us some comfort," she said.
"Foxhill was a big love of his, he adored that event. That was his place."
An event spokesperson said it was "very rare" for a non-rider to die at a Motocross event.
Series promoter RHL Activities said: "Langrish MX Club, are saddened to announce the death of Simon Mitchell, a gentleman well known within the motocross community.
"The thoughts and condolences of everyone concerned are with Simon's family and friends at this time."
Gareth Hockey, the event's promoter, added: "It is a massive tragedy. Simon was a popular guy around here."