Passengers were treated to a Japanese drum ceremony at the gate, along with cake and a sake toast prior to boarding. The flight pushed back a few minutes ahead of schedule and taxied to the runway with a water cannon salute sendoff.
The daily flight to Tokyo's Narita International Airport was originally scheduled to begin March 31, but was delayed because of the worldwide grounding of all Boeing 787s over concerns about the safety of its battery system.
United resumed flying the 787 domestically on March 20 after the Federal Aviation Administration approved a battery fix and lifted the grounding.
(Read More: United 787 Makes Uneventful Return to the Skies)
"This new nonstop service between Denver and Tokyo will build a new bridge between Colorado and all of Asia, strengthening our global connectivity and elevating the entire Denver area onto the international stage," said Michael B. Hancock, Denver's mayor, in a prepared statement. "We thank United Airlines for its partnership on this new frontier of opportunity and its commitment to the Denver market."
Seth Miller, a frequent traveler and loyalty program analyst from New York who authors the Wandering Aramean blog, booked his ticket months ago and was among the inaugural passengers.
"I am always looking for opportunities to have unique travel experiences and inaugurals are a good opportunity for that," Miller said. "So much of air travel has become boring, rote repetition; flights like this break that up and provide a refreshing change of pace."
Monday's flight was Miller's third onboard a 787 and he had absolutely no qualms about the jetliner's safety.
"I have no concerns about making the trip and see no reason I should," he said. "Air travel remains incredibly safe, far more than even my shuttle ride to the airport this morning."
(Read More: Onboard United's Inaugural 787 Dreamliner Flight)
United's nonstop flight departs Denver daily at 12:35 p.m. and arrives in Tokyo at 3:30 p.m. the following day. The return flight departs Tokyo at 5:25 p.m. and arrives in Denver at 1:15 p.m. the same day.
Monday's flight is the intended use of the long-range Dreamliner for United. Additional international 787 routes for the airline beginning this summer include Houston-London, Houston-Lagos, Los Angeles-Shanghai and Los Angeles-Tokyo.
—By CNBC's Darren Booth.