In 1977, Mostert became a teacher at Highveld Elementary School in Johannesburg, South Africa. He also taught at Mondeor High School in Johannesburg before moving to Alabama for graduate school. From 1985 to 1986, Mostert was a special education teacher at Old Dauphin Way School in
Mobile, Alabama. He served as Program Director and Director of Education at the Charter Southland Hospital in Mobile, Alabama from 1986 to 1987 and served two years as principal at the St. Mark School, Mobile, Alabama before pursuing his doctorate. In 1992, Mostert became the Program Coordinator for Programs and Licensure in Learning Disabilities at
Minnesota State University Moorhead,
Moorhead, Minnesota. He remained at the university, becoming associate professor, then professor of special education, until 2000. From 2000 to 2002, Mostert served as associate professor of Special Education at
Old Dominion University, Norfolk, Virginia. He left Old Dominion University for Regent University in 2002, where he continues to serve as Director, Special Education Doctoral Cognate and Professor of Special Education. In 2007, Mostert became Director of the Institute for the Study of Disability and Bioethics at Regent University. == Useless eaters and assisted suicide==