Ernst Mandel came from Bad Dürkheim in what was then the Bavarian Palatinate. His parents were David Mandel and Frieda Mandel, née Löb. He attended the Progymnasium in Bad Dürkheim and the Gymnasium in Neustadt an der Haardt until he passed his A-levels. In the summer of 1906, he began studying law, initially in Munich until the summer of 1909, interrupted by the winter semester of 1908-09 in Strasbourg. In the winter semester of 1909-10, he continued his studies at the University of Erlangen, where he also passed his first state examination in 1910. This was followed by a traineeship at the Bad Dürkheim district court, a year's military service in 1910-11 and further training at the Bad Dürkheim district court and Munich regional court. In 1912, Mandel was awarded his doctorate at the University of Erlangen with a dissertation entitled "The so-called external conditions of criminal liability under current law and under the preliminary draft". He completed his studies with the assessor exam. He probably did military service during the First World War.
He began his banking career in 1920 as an board secretary at Süddeutsche Disconto-Gesellschaft in Mannheim. Founded in 1905, the subsidiary of Disconto-Gesellschaft in Berlin was a successful regional bank in Baden and the Palatinate. In 1923, he moved to the parent company in Berlin, succeeding his boss, management board member Theodor Frank, who had been promoted to the circle of partners, the highest management body of Disconto-Gesellschaft, in 1922. Mandel also remained Frank's close collaborator in Berlin. In 1928, he was appointed deputy director of Disconto-Gesellschaft and, together with Oswald Rösler and Karl Ernst Sippell, he was regarded as one of the "crown princes" of the circle of partners. In 1929, in the course of the merger with Deutsche Bank, Mandel was appointed one of the 13 deputy board members of the new "Deutsche Bank und Disconto-Gesellschaft". At the end of 1931, the office of deputy board member was abolished in response to the banking crisis and in order to simplify management. Since 1932, the previous members of this circle - including Mandel - had held the title "Directors of the Bank" and had general power of attorney.
Mandel held numerous supervisory positions, including chairman of the "Revision" Treuhand-Aktien-Gesellschaft, deputy chairman of Adler Kaliwerke, Bergwerkgesellschaft Hope, Kaliwerke "Adolfs Glück" and Schnellpressenfabrik Heidelberg AG.
Mandel remained in his position until the beginning of 1937, when Deutsche Bank und Disconto-Gesellschaft concluded a cancellation agreement with him due to his Jewish origin. He received the sum of 172,500 Reichsmarks as compensation for his contractual claims. His emigration to the United States had already been prepared. On 13 April 1937, he, his wife Elise, née Crailsheimer (30 September 1893 in Strasbourg - 21 April 1981 in Scarsdale) and their children arrived in New York. The family settled in Scarsdale in the state of New York. Mandel was naturalized as a citizen of the United States of America on 27 December 1944. In 1952, he began a friendly exchange with the successor institutions of Deutsche Bank in West Germany, which only ended with his death in 1978. Mandel assisted the author of the anniversary book "100 Years of Deutsche Bank, Fritz Seidenzahl, with brief descriptions of the leading figures at Deutsche Bank and Disconto-Gesellschaft during the years of the world economic and banking crisis.

01.04.1920 - 1923 Süddeutsche Disconto-Gesellschaft, Mannheim, board secretary
1923 - 1928 Disconto-Gesellschaft, Berlin, deputy director
1928 - 31.10.1929 Disconto-Gesellschaft, Berlin, director
01.11.1929 - 31.12.1931 Deutsche Bank und Disconto-Gesellschaft, Berlin, deputy member of the management board
01.01.1932 - 31.03.1937 Deutsche Bank und Disconto-Gesellschaft, Berlin, director with general power of atttorney