During childhood walks in the woods and along the banks of the Big Muddy River, he recalls mentally noting the diverse flora and fauna around Murphysboro, Illinois.
"[2] Mohlenbrock's first project was presented at a 1948 science fair, called "Field Survey of Trees" of southern Illinois, completed with Kenneth Stewart.
As a master's student under John W. Voigt, he completed his thesis on "The vegetation of Giant City State Park: a floristic and ecological study" in 1954.
[5] Soon following the completion of his PhD work, Mohlenbrock was hired at Southern Illinois University Carbondale to fill in for a single quarter for a faculty member who had taken an unexpected leave.
Mohlenbrock's department at SIU provided one of the few places in the country that encouraged students to conduct floristic studies in pursuit of a PhD.
[6] Mohlenbrock advised 90 graduate students during his tenure at SIU and has received numerous awards in recognition of his instruction.
In 1985 he was named Chairman of the North American Plant Specialists Group of the Species Survival Commission of the International Union for the Conservation of Nature, a position he held for 15 years.