Kongshaug was born in Trondheim as a son of guitarist John Kongshaug. During his childhood and adolescence, he began to play the accordion (1950), guitar (1958) and bass (1964). Kongshaug got his examen artium in 1963, and trained in electronics at the Trondheim Technical School in 1967. Then he worked for Arne Bendiksen Studio (1967–1974) and Talent Studio (1974–79) in Oslo, and undertook some jobs in New York. In 1984, he founded his own recording studio, Rainbow Studio in Oslo and evolved into being one of the grand masters of Sound engineering. Altogether, he produced over 4,000 records, and was particularly known for some 700 recordings for ECM Records made from 1970 onwards.
Kongshaug played with Åse Kleveland winning the Norsk Melodi Grand Prix in 1966, and was third in the Eurovision Song Contest (1966). He has also played on dozens of recordings, including with Asmund Bjørken, Frode Thingnæs, Sven Nyhus, Arild Andersen and Frode Alnæs.