The worker "inappropriately" entered a restricted zone near the roller coaster around midday, according to an email statement issued by La Ronde's company parent Six Flags.
The incident is now under internal investigation.
"We don't know why the employee went in to the restricted security zone, which is locked. We have very strict procedures for these restricted zones," La Ronde spokeswoman Catherine Tremblay told reporters.
"We're trying to understand why those procedures weren't followed."
Ambulance services arrived at the scene around 1:30 p.m. but there were no efforts to revive the employee, given the obvious extent of his injuries.
Rescue workers treated six people at the site for shock, including one person who was sent to hospital.