His parents were Gábor Fáy, deputy governor and royal councilor, and Zsuzsa Diószeghy. He was baptized into the Reformed Church but converted to Catholicism in 1736. He studied philosophy in Nagyszombat, then entered the Jesuit Order in Vienna on November 8, 1736. After his probationary period, he completed teacher training in Szakolca. From there, he moved to Nagyszombat in 1740 to complete his studies in philosophy. In 1743–4, he taught in Sopron, followed by one year each in Nagyszombat and Győr. He began his theological studies in Vienna in 1747 and continued them in Košice. He was ordained a priest in 1748. In 1753, he went on a mission, where he succeeded in securing peace with the Guanajara Indians under the terms of the “missionary government” in effect in the Marañon and Gran Pará. The Portuguese governor, Francisco Xavier de Mendonça Furtado—brother of Minister Pombal—classified this agreement as a “rebellious political peace treaty” and attacked the mission. He wrote a letter to the Portuguese queen, Maria Anna of Austria. However, the Portuguese intercepted his letter and forwarded it in a falsified form. On this basis, Fáyt was accused of rebellion and high treason. In 1757, he was captured and taken to Portugal. There, he was first exiled to Roriz. In 1759, he was imprisoned in Almeida, and from there, in 1762, he was transferred to the São Julião da Barra fortress in Lisbon, where he died. Béla Bangha, S.J., writes extensively about his life.

Regia Seren. Archiducis Josephi indoles. Nagyszombat, 1745.