At 18.20 hours on 16 April 1944, U-407 fired a spread of three torpedoes at three overlapping steamers in convoy UGS-37 about 17 miles off Derna and heard two detonations. The Meyer London in station #12 and Thomas G. Masaryk in station #23 were hit by one torpedo each.

The Thomas G. Masaryk (Master Robert Harrison Sloan) was hit on the port side at the #3 hatch, tearing a hole of 26 feet in the side plating. Immediately the acetone in the hold caught fire and ignited the magazine for the 20mm guns. The flames and exploding ammunition forced the men from the bridge so the ship went out of control and began to circle to starboard, missing the other torpedoed ship by only 20 feet. The eight officers, 35 crewmen and 27 armed guards (the ship was armed with two 3in and eight 20mm guns) abandoned ship in one lifeboat and three rafts, were picked up by HMS La Malouine (K 46) and landed in Alexandria on 23 April.

The burning Thomas G. Masaryk was towed by HMS Captive (W 148) into Maneloa Bay, Libya and beached there. She was then shelled in order to extinguish the fire and sank in 28 feet deep water. About 50 percent of her cargo was salvaged by the American Liberty ship William M. Meredith with the help of British troops and discharged at Port Said. Later the refloated vessel was towed by the same tug to Port Said where the remaining cargo was salvaged and the ship declared a total loss.