In 1971 Garmatis was appointed President of Hellenic College and Holy Cross School of Theology in Brookline, Massachusetts, while remaining as administrative overseer of the Diocese of Detroit.

He simultaneously served as the Bishop of the New England area during his five-year tenure as President of Hellenic College and Holy Cross School of Theology.

Garmatis was elected to the episcopacy of the Greek Orthodox Church by the Holy and Sacred Synod of the Ecumenical Patriarchate of Constantinople in 1969.

He was enthroned by Archbishop Iakovos of the Greek Orthodox Archdiocese of North and South America on May 1, 1979, as the Bishop of Chicago at the Annunciation Cathedral.

In November 1997, the Holy Synod of the Ecumenical Patriarchate elected Garmatis to the active Metropolitanate of Chicago and Exarch of Ionia.