Officials say the fire began in a hay field being used as a parking lot for the festival and quickly swept through 34 vehicles Sunday afternoon.
Damage is estimated a more than a million dollars.
Niagara-on-the-Lake Fire Chief Rob Grimwood says it was one of the fastest-moving fires he has seen in his career.
Grimwood said 19 vehicles were totally destroyed and another 15 damaged.
Officials don't know what started the fire.
However, Grimwood says given the dry conditions it could have been a discarded cigarette or a car's exhaust coming into contact with the hay.
There were no injuries, but some firefighters were treated for smoke inhalation at the scene.
Happening right now in Niagara on the Lake. Grass fire sparked multiple car fire! Please send help asap!!!@TheSpec @CBCNews @CBCAlerts pic.twitter.com/fzXHt9Y4q8
— Denise van Es (@van_EsDenise) July 8, 2018