Alexanda Anon Kotey, 37, pleaded guilty to all eight charges against him at a hearing in the U.S. District Court in Alexandria, Virginia.
The charges include hostage-taking resulting in deaths and the provision of material support to the Islamic State from 2012 to 2015. This is reported by AP.
He pleaded guilty to killing four American hostages - journalists James Foley and Steven Sotloff, aid workers Peter Kassig and Kayla Mueller - and European and Japanese citizens who were also held captive.
The members were called The Beatles
Kotey is one of four IS members who were called "The Beatles" by their prisoners because of their British accents.
He and another man, El Shafee Elsheikh, were taken to the United States last year to be prosecuted after the United States assured Britain that none of them would be sentenced to death.
Elsheikh is still scheduled to stand trial in January.
A third Beatle, Mohammed Emwazi, also known as "Jihadi John", was killed in a drone hunt in 2015. A fourth member is serving a prison sentence in Turkey.
No chance for freedom
The agreement stipulates a minimum sentence for life without conditional freedom.
After 15 years, however, Kotey would be entitled to be transferred to the UK to face possible prosecution there.