The decision to divert the plane was made after its crew reported an emergency. Passengers on board said there was smoke in the plane's cabin.
United Flight 1181 landed about 8:56 a.m. at the Ontario airport, where it was met by police and firefighters, airport spokesman Harold Johnson said. The Boeing 757, with 206 people aboard, landed without incident.
The airline said the flight crew reported a burning smell.
"Our maintenance team is reviewing the aircraft and we're working as quickly as possible to get our customers to Washington, D.C.," they said.
The passengers were escorted off the plane to an airport terminal.
According to FlightAware, the plane departed from Los Angeles International Airport at 8:25 a.m. and was destined for
Passenger Mitchell Hashimoto tweeted about the experience, reporting light smoke inside the plane.
He said the plane was cruising, then quickly had to make an emergency landing. It all happened within five minutes.
Hashimoto described the experience as uncomfortable.
"But also, our pilot handled that situation super fast and professionally. High five," he tweeted.
Hashimoto had planned to travel to Brussels, but he reconsidered.
"I don't think I'll be flying to Belgium today," he tweeted. "One flight is good enough for me today. Will try again another day."