Dejaegere studied tropical agricultural engineering at the Agricultural Institute in Gembloux. He then studied botany at the ULB, where he obtained his doctorate in 1967. He was assistant to professors Homès, at the time rector of ULB, and Van Schoor.
In 1969, he became a lecturer at the ULB but soon switched to the newly established VUB. There, with Prof Neirinckx, he set up a greenhouse laboratory. His scientific activities mainly focused on the biochemical aspects of plants: their nutritional processes, their productivity and their use of biomass. He maintained a special relationship with the University of Sydney (Australia), where he was a visiting professor in the early 1980s. Dejaegere was dean of the Faculty of Science from 1986 to 1989.
He became rector in 1991. He was rector during the educational reforms of CVP minister Coens. The universities opposed the savings and Dejaegere participated in the first ever demonstration of scientists in Belgium. In addition, Dejaegere was committed to freedom. VUB was a young university and still developing, with independence from the ULB on the agenda. He tried to implement changes in a progressive way and the students often found a supporter in him. Under his rectorate, the system of educational evaluations was also structurally reformed.
As a personal highlight of his rectorate, Dejaegere mentioned the presentation of the honorary doctorate to Nelson Mandela. Out of gratitude for his services, the university awarded him the title of emeritus professor when he retired in 1996.