Father Kassovic was born on February 18, 1897, in Bosaca, Czechoslovakia, to John and Eva Kassovic. He had eight siblings, two of whom became priests and two of whom became nuns.

He was ordained a priest on June 29, 1934, in Europe. He joined the Diocese of Peoria in 1946 and served at various parishes in the state of Illinois until his retirement in 1977. He was an assistant pastor at St. Roch Church in La Salle, and then served as pastor of St. Anne Church in Spring Valley from 1953 to 1969. Later, he served as a hospital chaplain at St. Margaret’s Hospital for five years.

While serving at St. Anne’s Parish, he played an active role in the design and construction of the new church and its parish building along U.S. Routes 6 and 89.