In 1941, he graduated from Watertown High School, where he enjoyed performing in musicals and Gilbert and Sullivan shows and excelled academically, gaining membership into the National Honors Society.
At Boston University, he majored in physics and graduated cum laude in 1946. He continued onto graduate school at MIT.
Professionally, he developed and manufactured the ultraviolet microscope at Scientific Specialties Corporation. At Northrop Corporation he developed the radiometric sextant and Type 11 periscope, which were used for the navigation systems of atomic submarines. At Itek Corporation and Honeywell he developed cameras for US satellites for earth photography. Even in retirement, Koumjian developed a computer software office system for his daughter’s optometry practice.
In addition to succeeding professionally, he held many leadership positions in the community. In the 1970s, he was president of the National Association of Armenian Studies and Research and in the 1980s he held the position of Commander of the Knights of Vartan.
He also soloed in the St. James Armenian Church Choir of Watertown from 1958 to 1973. He was the lead male in the Armenian musical Kharabaghee Melikner in 1960 and recorded the Divine Liturgy of the Armenian Church as the tenor soloist under Dr. Arra Avakian in the 1960s.