Born in Adelaide, South Australia, on 20 June 1934,[1] Johnsson earned a Bachelor of Technology (BTech) degree in chemical technology from the University of Adelaide (1954) before attending Avondale University, where he received a Bachelor of Arts (BA) degree in theology (1959) and met his wife Noelene.

His PhD dissertation, written under the direction of Leander Keck, was entitled "Defilement and Purgation in the Book of Hebrews."

Johnsson was a missionary to India at Vincent Hill School and Spicer Memorial College (1960–1975).

After retiring in 2006, he served for seven years as part-time assistant to the General Conference President for Interfaith Relations.

Johnsson, who wrote more than forty books and 1,000 articles, received the following honorary degrees: Doctor of Divinity from Andrews University (2007)[3] and Doctor of Humane Letters from Loma Linda University (2016).