When the Germans invaded Norway on 9 April 1940, "Prins Olav" was in the northern port of Harstad, having begun her north-bound route from Bergen on 1 April. Being in the part of Norway which initially avoided being occupied, "Prins Olav" was requisitioned to assist with the Norwegian mobilization effort. Serving as a troop ship from 11 April, she first transported Norwegian troops from the Lofoten region to Sørreisa in Troms, and later from Kirkenes in eastern Finnmark to Gratangen near the front lines at Narvik. In the latter mission "Prins Olav" transported two companies of the 1st battalion of Infantry Regiment 12, the rest of the battalion having been shipped out several days earlier on the "Hurtigruten" ships and "Kong Haakon", and the steamer "Hestmanden". For the last stretch of their voyage, on 17 April, the Norwegian transports were escorted by the Norwegian patrol boat "Heimdal".