He practiced accountancy with Deloitte, Plender and Griffiths, was admitted to the New York bar in October 1915, enlisted in the U.S Army 60th Field Artillery in 1916 as a private/sharpshooter, was honorably discharged in 1918 as a sergeant Quartermaster Corps after serving as associate counsel on the Provost Marshal General Staff.

He served as counsel for the Housing and Health Division of the War Department in 1919, and was a member of the law firm Guggenheimer, Untermyer and Marshall from 1920 to 1933.

He was the author of numerous publications: Directorships: Fruit & Produce Acceptance Corp., Lessing's Inc., Louis Phillipe Inc., Leopold Stern and Sons Inc., Affiliated Products Inc., United Steel and Tube Inc., Neet Inc., G.R.

Kinney & Co., Jean Patou Inc., Immac Inc., Encyclopædia Britannica Inc., Member of numerous organizations: American Bar Association/ Atlantic Beach Club/ Bankers Club of New York/ Bar Association of the City of New York/ Columbia University Club/ Columbia Varsity Club/ Democratic National Finance Committee/ Executive Finance Committee of the Democratic National Campaign Committee/ Federation of American Zionists and the American Zion Commonwealth/ National Democratic club/ Member, President Roosevelt's pre-convention campaign committee/ Pi Lambda Phi In 1941, immediately prior to Germany's bombing assault on Moscow and with only six hours notice, Steinhardt sent his wife, daughter and her governess, each carrying only one suitcase, out to Stockholm on one of the last planes departing with families of diplomats to any safer destinations within Europe.

In July of this period, the famed photographer, Margaret Bourke-White, managed to get into Moscow to the embassy residence so as to document the German bombings.

Her portfolio assignment produced startling visual imagery for the American public in the issue of Life Magazine, Vol 11, No 9., pp. 15–21.

His mission directives included buying up all the available sources of chrome the Nazis needed for the manufacture of steel for their war machine.

While Ambassador to Turkey, Steinhardt, in part due to his Jewish heritage, played a significant but not openly known role (due to his public diplomatic position) in numerous Jewish- related refugee transit evacuations: the rescue of Hungarian Jews from Bergen Belsen, Jewish children from Romania, and many eminent intellectuals fleeing Europe to find refuge in Turkey, Palestine, and the United States.

Steinhardt and Ira Hirschmann of The War Refugee Board secured leaky boats wherever possible with no assurance of the safety of such vessels but the hope that anything might succeed if tried.

On August 1, 1943, Operation Tidal Wave, the air attack by U.S. Army Air Forces based in Libya and Southern Italy, began their daring strategic bombing mission over the Ploiesti refineries and oilfields in Romania in order to deny the German war machine of needed fuel.

Of the 53 aircraft and 660 crew members lost, many had little or no fuel to return to their bases, so they were instructed to ditch in neutral Turkey where there was a substantial American presence to assist and on which to fall back.

Many a dark unlit night following the bombing raid, Steinhardt and his daughter, in a jeep with a small convoy of his embassy military personnel, sortied out into the wilds and deserts attempting to locate and return wounded American pilots, destroy the bombsights so the Germans would not know of their existence, return with the wounded, secretly patching them up by the embassy medical team and moving them quietly through channels back to their points of origin.