He served with the Danish Merchant Navy for four years and on an American oil tanker for another two as a radio operator.

in Electrical Engineering at the University of British Columbia (UBC) in 1960 and subsequently obtained practical industrial experience with the BC Energy Board and Chemcell in Edmonton.

In July 1966, he returned to University of Alberta as assistant professor and spent 27 years teaching, researching and taking a large load of administrative responsibilities for the Department of Electrical Engineering.

In co-operation with his wife Penny, a professional genealogist, he wrote his autobiography, the history of his parents' 172 descendants and a volume of translations of historical articles on Læsø.

At the age of 75 Christensen took up an adjunct professorship in the School of Engineering Science at Simon Fraser University in Burnaby, BC, where he had an office, a computer and a grad student – but no salary.

During this time he published a number of papers in the field of asymptotic stability of linear and nonlinear systems, a continuation of his Ph.D. work.

Another development was the solution of the least absolute value estimation problem originally posed by Laplace in 1750 by using linear programming.