He lived in New Delhi, India for a year, where he studied tabla with Vijay Ateet, incorporating Indian classical music concepts into his original writing.
In 1993, Armacost moved to New York and recorded his debut album, Fire (Concord Jazz), with Kenny Barron, Gerald Cannon, Billy Hart, and Shingo Okudaira [de], followed by a live recording (Live at Smalls) with guest soloist Tom Harrell.
During this time he also worked with Kenny Barron, Roy Hargrove, the David Murray Big Band, Maria Schneider Orchestra, Randy Brecker, Al Foster, Jimmy Cobb, Victor Lewis, Peter Erskine, Don Friedman, and Joris Teepe.
In the 2010s he was also a member of the New York Standards Quartet and co-leader of the Brooklyn Big Band, for which he composes and arranges.
He recorded an album with Mike Rodriguez [de] and Yutaka Shiina titled Future Swing (2015).