His first academic appointment was at Upsala College, East Orange, New Jersey, where he served as Instructor in Fine Arts (later Assistant Professor) and Director of the Upsala Choirs from 1966 till 1972. He then accepted a position at the University of Colorado in Boulder, where he served as Assistant Professor, later Associate and Full Profesor of Composition, and as the Director of the University's Electronic Music Studio, New Music Festival, and New Music Ensemble. He has served as Chair of the Composition/Theory Faculty at Colorado from 1984 to 2001.

Toensing has won numerous awards for composition, including the Joseph H. Bearnes Prize from Columbia University, two BMI student composer awards, a Guggenheim Fellowship, and a commission from the National Endowment for the Arts. He has been a MacDowell Colony Fellow three times.

His compositions span a variety of styles, from the gestural, free atonal work of the 60's and 70's to a renewed interest in various forms of diatonic music in more recent years. He has written numerous works for chorus, chamber music in various genres, and works for large ensembles. His music has been influenced by Russian Orthodox chant, Lutheran chorales, and also by the works of other composers: Schuetz, Gesualdo, Varese, Finney, and Paert.