Born into a Jewish family[1] in Stuttgart, Württemberg, he was a son of Max Mannheimer, a wine merchant, and his wife, Lili Sara Fränkel.

According to an obituary of the banker published in the 21 August 1939 edition of Time: During the War, barely out of college, he got a job in the German Government bureau directing the flow of raw materials through Germany.

As such it helped the German industrialist Robert Bosch to establish an American firm, thus circumventing the rules against German capital in the U.S.A. [3] The Time profile, while acknowledging Mannheimer's power and position, was also remarkably negative, describing him as "fat-lipped, mean, noxious, cigar-chomping"[2] man who gave one of his mistresses a gold bathtub and who, "after 20 years in The Netherlands, could not speak enough Dutch to boss his chauffeur.

[4] According to the Time profile, Mannheimer, who had long been obese and in ill health, suffered a heart attack during the wedding and had to be revived with two injections.

Contemporary observers quickly speculated on the manner of death, with many newspapers suggesting suicide; but Mannheimer's health had always been precarious, due, in part, to his excessive weight.

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."

The day after his death, Mendelssohn & Co. in Amsterdam announced that it was insolvent and that Mannheimer's collections, which were valued at more than 13 million guilders (approximately $100,000,000 today), had been largely funded on unlimited bank credit.

Housed at his homes in the Netherlands and France, Mannheimer's art (which included works by Chardin, Fragonard, Watteau, and Rubens, at least one fake Vermeer,[5] gold reliquary busts, tapestries, Meissen porcelain, and Judaica including a naturalistic circa-1800 Hanukkah lamp known as the "Oak Tree Menorah"), and his collection of 18th-century furniture (much of it acquired for him by the American decorator Elsie de Wolfe and the Paris decorator Stéphane Boudin) were seized by the bank.

"[This quote needs a citation] The entire Mendelssohn firm was forcibly liquidated by the Nazis later that same year—its assets were transferred to Deutsche Bank—and much of Mannheimer's art was purchased by Adolf Hitler in 1941, after the German invasion of the Netherlands.