He attended Napier Boys' High School and left early to study composition and piano at Victoria University of Wellington.

Psathas studied further with composer Jacqueline Fontyn in Belgium before returning to New Zealand, where he has since lectured in music at Victoria University and continued to fulfill a busy schedule of commissions.

[3] Psathas' collaboration with Evelyn Glennie has been long-standing and produced many commissioned works, including the 2001 double concerto for piano and percussion View From Olympus.

[3] A highlight of 2000 was the premiere of the saxophone concerto Omnifenix at an outdoor concert before an audience of 8000 people at the 2 Agosto Festival in Bologna, Italy.

[4] A retrospective concert of Psathas' chamber music was given in the 2000 New Zealand International Festival of the Arts, culminating with the premiere of the specially commissioned Piano Quintet.

[7] In 2003 a new CD of chamber works, Fragments, was released to critical acclaim and went on to win another Tui Award for Best Classical Album in 2004.

[14] In 2018 Psathas retired from his long university tenure to become a full-time freelance composer, and was granted the position of Emeritus Professor at the New Zealand School of Music.