After carrying out various exploits, on 12 Aug 1944, Marholz was ordered to prepare his boat for evacuation to Norway due to the onslaught of allied forces into Brittany.

Orders were for all U-Boats that could make the journey to Norway to do so and to destroy all U-Boats deemed unfit or unable to make the voyage. Marholz loaded up with as much fuel oil as he could carry, vital spares, food and torpedoes, and departed for Norway on 29 Aug 1944, carrying out a five day patrol in the North Channel en route. He suffered an airborne attack during this patrol and he aborted the patrol and headed for Norway, arriving in Bergen on 9 Oct 1944.

Repaired, her next patrol was from Bergen to the North coast of Scotland departing on 8 Feb 1945.  She patrolled the North coast up to the Minches Channel with no success until Loeder, her new Commander, decided to head south into the Moray Firth where he may have more luck.  The Moray Firth had much merchant shipping transiting from Aberdeen in the south and Invergordon to Scapa Flow and the pickings should be good.

On the 16 Feb 1945, she was shadowing Allied Convoy No. WN 74 into the Moray Firth when the Canadian Frigate HMCS St. John detected her.  The St John attacked U-309 and the first attack on her produced some light oil on the surface.  Two further attacks were carried out using the Hedgehog antisubmarine mortar weapons system with further oil but the fourth attack using depth charges produced wreckage including charts, signal books and cork insulation material.  The boat was lost with all hands.