By 9 p.m., the fire had burned 175 acres and was 50 percent contained. Officials said the evacuation of 200 homes had been lifted, except for homes on Hidden Valley Avenue, Alta Vista Avenue, Briarcliff Road and Highland Place.
Fire officials said they hoped to have the fire fully contained sometime Sunday.
Firefighters responded to a home near the area of Hillcrest Boulevard and Madison Avenue around 11:06 a.m. Officials said the fire was sparked by a gardener clearing brush in a home near that intersection. The flames then spread to the undeveloped hillside behind the residence.
City spokeswoman Jennifer McLain said mandatory evacuations were ordered as a precaution along several streets, including Heather Heights Court, Magnolia Avenue, Alta Vista Avenue, Melrose Avenue, Patrician Way, Hidden Valley Avenue, Mesa Circle, Sky Way, Crescent Avenue, Highland Place, Franklin Place, Scenic Drive, Cloverleaf Drive and Ridgeside Drive.
Temporary shelter was set up at the Monrovia Community Center at 119 West Palm Ave.
Highly-visible flames and smoke snarled traffic on the nearby 210 Freeway.
Firefighters called in a fixed-wing aircraft to help fight the blaze. More than 200 firefighters and five helicopters were at the scene. Winds were light, but firefighters braced for any shift in wind conditions. Fire officials said the brush had not burned in 55 years.
Officials said a Pasadena firefighter suffered a heat-related injury, but was treated at the scene and was doing fine. No property damage has been reported.
The Associated Press contributed to this report.