Nils was raised on Long Island, New York. Soon after receiving an undergraduate degree in psychology with an emphasis on persons with disabilities, he and his wife, Kathleen, worked as houseparents for young women with emotional and behavior problems.
Nils then joined the staff of the Austin (Texas) State School, an institutional facility for individuals with mental retardation. While there, Nils helped change the roles of the institutional staff, allowing residents greater freedom to actively participate in decisions regarding their daily lives. He moved on to work with students identified as deaf/blind while also pursuing a master of arts degree in Special Education.
From 1976 through 1987, he served as assistant director of community-based services at the Infant-Parent Training Program. He continued to work while pursing his doctoral degree at the University of Texas at Austin. In 1988, Nils joined the research staff at PRO-ED. In 1993, he became director of research, a position he held until 2002. After leaving PRO-ED, he became an independent author and spends his days developing new tests and revising existing instruments in the area of disability.