Retirement had been scheduled for the end of 2018. The United fleet currently includes 20 747-400s seating 374 passengers.
"Farewell to the queen of the skies," United President Scott Kirby wrote Wednesday in a letter to employees.
"As deeply connected as we all are to this iconic aircraft, the time has come to retire our 747 fleet from scheduled service," Kirby said.
"It's a bittersweet milestone -- this jumbo jet with its unmistakable silhouette once represented the state-of-the-art in air travel," he said. "Today, there are more fuel-efficient, cost-effective and reliable widebody aircraft that provide an updated in-flight experience for our customers traveling on long-haul flights."
In 1970, Pan Am became the first airline to operate the 747. United followed the same year, putting the aircraft into operation between California and Hawaii.
But with the development of twin engine alternatives including the 777 and more recently the 787, the economics of operating an older, four-engine 747 have become far less favorable.
Delta (DAL) , which operates five 747s, is the only other U.S. carrier that still flies the aircraft. It too plans to retire the aircraft by the end of this year.
Kirby promised "an unforgettable retirement celebration."