The fire broke out near The Old Road and Skyview Lane at about noon.
By late afternoon, it had grown to about 50 acres, but fire officials said they had contained the forward progress and brought it down to a two-alarm blaze.
Containment was estimated at 50% by 9:30 p.m., with firefighters remaining on scene overnight to continue increasing containment amid the cooler temperatures and lighter wind.
Large plumes of smoke were billowing in the area, created air-quality issues for visitors and particularly those with asthma.
Twelve people were transported to the hospital because of smoke and heat issues.
Six Flags Magic Mountain, Hurricane Harbor evacuated after brush fire erupts near theme parks
"I was working the gate and it got smokier and smokier," said park employee Richard Waldron. "It became a respiratory issue. There was ash falling around."
The offramp on the soutbound 5 Freeway at Magic Mountain Park was ordered closed for 90 minutes as firefighters battled the fire.
L.A. County Fire later asked all guests to remain at the park as exit roads were temporarily closed. Exit roads were later opened shortly before 2 p.m. and the park was evacuated and closed for the day.