Truant had a relatively active career serving in the main naval theatres of war – Home waters, the Mediterranean, and the Pacific Far East.

Truant's first major victory came when she torpedoed and damaged the German light cruiser Karlsruhe off Kristiansand, Norway, which disabled both engines and power stations.

As well as 8,000 tons of wheat, she had on board the captain and 22 survivors of the British SS Haxby, which had been sunk by the raider, as well as her own Norwegian crew.

[3] She almost sunk a large liner, but held off as poorly displayed hospital markings were spotted (possibly the Hikawa Maru).

A few days after she engaged in a gun battle with the Japanese army cargo ship Tamon Maru No.1, which sank shortly after.