He graduated from St. Anthony's High School and then attended Adelphi University from 1990 to 1993, amassing 17 goals and 15 assists over his collegiate career.

It was with the 1998 Fire team that Armas emerged as an exceptional player, helping them win their first MLS Championship that year.

In 2019, the Red Bulls finished sixth in the Eastern Conference and were eliminated by the Philadelphia Union in the first round of the playoffs.

The team was in the middle of a scoring slump when Armas parted ways with the club on September 4, 2020, the day after a 1–0 loss to D.C.

[9] In 2021, Armas was hired as head coach of Toronto FC, replacing Greg Vanney, who departed the club at the end of his contract.

[11] On December 7, 2021, Armas was hired by Manchester United as assistant coach to work under interim manager Ralf Rangnick.

Armas was named co-interim head coach alongside Michael Skubala and Paco Gallardo following the sacking of Jesse Marsch in February,[14] the trio overseeing a 2–2 draw at Old Trafford with Manchester United on 8 February 2023[15] and Skubala acting as solitary interim manager in the home loss against the same opponents four days later.