Born in Aitkin, Minnesota, Dehning moved to Los Angeles in 1956 where he attended high school. He was first admitted to UCLA's engineering school, but eventually graduated with a Bachelor of Arts degree in Music. At UCLA, Dehning sang with Roger Wagner and Paul Salamunovich. Immediately following, Dehning enrolled at the University of Southern California, where he attained master's and doctoral degrees in church and choral music with highest honors. While there he studied with Charles C. Hirt, James Vail, Ingolf Dahl, Walter Ducloux, and Halsey Stevens. He also taught conducting and founded a chorus as an assistant in the department.

Dehning began his work as a full-time music professor in Marquette, Michigan, having been hired as Director of Choral Activities at Northern Michigan University in 1970. Shortly thereafter, he was hired by the University of the Pacific in Stockton, California, to head their choral program in the music conservatory there. He and his family spent twenty years in northern California (1972-1992).

In 1980, Dehning took his first sabbatical in England, where he observed and sang with John Alldis and the London Symphony Chorus and observed the John Alldis Choir. In 1988, Dehning took his second sabbatical in Germany where he lectured at Munich’s Hochschule für Musik and spent time observing university and professional choruses in Munich and Stuttgart, including the Bavarian Radio Chorus and Helmuth Rilling's Gächinger Kantorei. Following a performance at an ACDA national convention with the California Choral Company in 1991, Dehning was asked by Morten Lauridsen, famed choral composer and Professor of composition at the University of Southern California, to audition for the position of Chair of Choral Music at USC. He accepted the position and went there in 1992.

His tenure at USC included numerous ACDA convention appearances, tours, and international competitions with the renowned USC Thornton Chamber Choir (originally named the USC Chamber Singers by its founder, Charles C. Hirt). While at USC, he resumed a long-standing relationship with Helmuth Rilling, preparing a number of choruses for Rilling's appearances in Los Angeles. In addition, he also prepared choruses for the Los Angeles Philharmonic and Los Angeles Chamber Orchestra. Dehning became a frequent visitor to South Korea--a country that cherishes choral music--where he has appeared often as guest conductor of professional choruses and clinician with numerous universities. In addition to appearances with professional choruses in Korea—including the National Chorus, the Bucheon City Chorus and the award-winning Pilgrim Mission Choir—he also worked as guest conductor of the Karlovy Vary Symphony in the Czech Republic.

Dehning retired as Chair of the Choral Music department at the University of Southern California in 2007 after fifteen years there. In recent years, the graduate student body grew under his leadership. He recruited colleagues such as Dr. Magen Solomon and Dr. Nicolas Strimple to aid in the instruction of the students. Although a versatile conductor of both choirs and orchestras, Dehning also established a reputation with his students as an effective teacher. He continues to do clinics, workshops, and master-classes, and to make appearances as guest conductor in the U.S. and abroad. He currently lives in Huntsville, Alabama.