An airline spokesman said the action was taken after the pilot noted heavy vibrations coming from one of the engines of the Boeing 757 aircraft.
The plane was still about 100 miles east of Ontario when an emergency was declared. However, the aircraft landed safely at about 9 p.m. with no one being injured.
There were 190 people on board the flight.
The Delta spokesman said the airline is investigating to determine the cause of the mechanical problem.
City News Service and the Associated Press contributed to this report.