The US European Command announced its decision this evening to reduce personnel at Europe's largest surveillance and interception facility - on the A59 Harrogate to Skipton road - by October 2016.
In a statement it said: "The (US) Department of Defense is currently realigning its workforce around the globe by employing advanced technologies and combining similar mission activities worldwide to create more efficiencies.
"As a result, approximately 500 US military and civilians positions will be eliminated as the 421st Air Base Group and other US military units will be deactivated.
"Despite this reduction in personnel, US European Command remains committed to international security in the region and will continue to work in close cooperation with the United Kingdom."
The distinctive 33 white "golf balls" nine miles west of Harrogate contain satellite antennae capable of intercepting two million conversations an hour for Washington's National Security Agency.
It also provides data for the US missile defence system.
There are currently around 2,200 people stationed at RAF Menwith Hill - two thirds US, the rest Ministry of Defence.
Last summer parents at the US-run base were told its school would be closing while an application has been lodged to demolish 66 houses on site. The base's Burger King has also closed.
North Yorkshire county councillor Michael Harrison, who represents the area, said: "My thoughts first of all go to the people who are losing their jobs.
However, he added: "The base contributes significantly to the local economy, not only the immediate area but Harrogate district.
"It appears to have a future but the job losses will make a big dent in the economy."
He added that many of the military and civillian contractors lived off base.
Menwith Hill hit the headlines last year when whistleblowers Edward Snowden and J. Kirk Wiebe claimed it was used to listen in on the calls of German chancellor Angela Merkel and 35 other world leaders.
The RAF describes its primary mission as providing "intelligence support for UK, US and allied interests".
Lindis Percy, of the Campaign for the Accountability of American Bases, who has been jailed over her regular protests, said: "It is very significant and we absolutely welcome the news of movement by US forces back to their borders and I hope they leave the dales as they originally were."