Mohr joined the Reichsmarine in 1934. After serving as first Watch Officer (second-in-command) to Georg-Wilhelm Schulz in U-124 on three patrols, in September 1941 he assumed command of the U-boat on six patrols, and sank 27 merchant ships, for a total of 129,292 GRT of Allied shipping. This includes four ships from Convoy ONS-92 sunk on the night of 12 May 1942, totalling 21,784 tons. Mohr also sank the British light cruiserHMSDunedin and the Free FrenchcorvetteMimosa.