At 10.35 hours on 27 Feb 1942 the unescorted Macgregor (Master William George Todman) was attacked with gunfire by U-156 while steaming on a non-evasive course at 10 knots in fine weather about 15 miles northeast of Cabo Frances Viejo, Dominican Republic. On 26 February, the U-boat had no torpedoes left and the deck gun was damaged since the unsuccessful attempt of shelling an oil refinery on Aruba earlier during the patrol, so Hartenstein decided to repair the gun by sawing off the ruined portion of the barrel and to weld on counterweights. Not long after the crew had carried out the repairs, this ship was sighted and followed for 16 hours until moonrise. Macgregor was taken under fire from a distance of about 1600 meters on her starboard quarter and was first hit by a shell that struck the stern just ahead of the poop, blowing the hatch covers off #4 hold, killing one crew member and slightly wounding two of the three gunners on watch at the stern gun, who had to be replaced before they were able to fight back (the ship was armed with one 4in and five machine guns). The Germans opened fire with the 37mm AA gun at the bridge after distress signals were heard, but this enabled the DEMS gunners to open fire by aiming in the direction of its tracer rounds as it was too dark to see the attacker, firing seven or eight rounds in a period of 30 minutes. However, the U-boat ceased fire after the first shell was fired by the freighter and continued the attack quickly from the port quarter, a maneuver that was later repeated and gave the British crew the false impression that they were under attack by two U-boats. The ship tried to head for the nearby coast, but was hit in the #1 and #2 holds and the engine room and eventually incendiary shells set her on fire amidships after 35 minutes. The radio operator continued to send distress signals until the wireless room was struck by a shell and wrecked, however he and the master who was also in the room managed to scramble out without injury. The master then ordered the crew to stop the engines and to abandon ship while the radio operator took care of the severely injured chief officer who had lost a hand when his arm was struck by shrapnel. The falls of the port boat were severed by a shell that hit amidships while it was lowered and drifted away with only nine men in it. Miraculously no one was hurt and most survivors then left in the starboard boat. The master remained aboard to make certain that everyone had got away and was almost killed when a shell struck the stern where he was releasing a raft. However, he was not hurt when blown onto the #1 hatch, then jumped overboard and swam to raft which was shortly thereafter passed so close by the U-boat that he could hear them talking in German. At 11.30 hours, a vessel was seen to approach from the northwest and U-156 immediately left the area as it was certain that the burning ship was lost. In all, they had fired 92 shells from the deck gun and 112 rounds from the 37mm AA gun.

The Macgregor had developed a list to port after the engines were stopped and eventually capsized to port and sank by the head at 12.10 hours. Both lifeboats had splinter holes and were leaking, nevertheless the port boat searched the wreckage for survivors and found the master after about one hour. The severely injured chief officer had been inadvertently left behind during the abandonment because the radio operator went away to get a rope to lower the injured man into a boat and could not find him anymore when returning, assuming that others helped him to abandon ship, but the delirious chief officer climbed down an iron ladder underneath the gun platform where he lied down, slid off when the ship sank and then clung to a nearby raft. When the port boat found him a large shark was circling his raft and he was quickly pulled aboard. During the search so many sharks were present that they had to be pushed away from the side of the lifeboat with the oars, but none of the five men that were swimming in the water until being rescued had been attacked by them. At 15.00 hours, the master, 23 crew members and six gunners were picked up by a San Domingo Coast Guard cutter and landed at Puerto Plata, where eleven injured men were treated in a hospital.