On 9 August 1939, two months after his wedding, Mannheimer reportedly was in his Amsterdam office when he received a phone call from an unidentified party, left the office, boarded a train for France, and joined his wife at their house, where he died the same day.
Whatever the actual cause of death, The New York Times was just one of many leading world newspapers which reported that Mannheimer's demise took place under "circumstances attended by a measure of mystery."