Theissen earned a bachelor's degree in secondary mathematics education at the St. Ambrose University.
[1] He earned a master's degree and Ph.D. in educational measurement and statistics from the University of Iowa.
[3] He joined the New College of Florida in July 2016 as the president's chief of staff and a professor of statistics.
[1] On January 2, 2020, he became the director of institutional research at the University of Hawaiʻi at Hilo.
[4] On January 31 2023, Thiessen was appointed as the interim president of the New College of Florida by the board of trustees, succeeding Patricia Okker.