After a two-year interim during World War I, when he served in the American military, Marcus returned to graduate studies in Cincinnati.

After receiving rabbinical ordination in 1920, Marcus was appointed to the faculty of HUC, where he began teaching biblical history.

In 1965, he was appointed to HUC's Milton and Hattie Kutz Distinguished Service Chair in American Jewish History.

In the 1970s, while director of the American Jewish Archives, Marcus publicly shared the account of Regina Jonas in response to the ordination of Rabbi Sally Priesand in 1972.

Marcus informed The American Israelite in July 1973 that the only other known Jewish woman to receive ordination was Regina Jonas of Berlin but that she perished at the hands of the Nazi regime.