He received his Ph.D. in 1935 with a dissertation, supervised by Schaeder, on Palmyrenian inscriptions (Die Sprache der Palmyränischen Inschriften).
After teaching for a year in Florence, Italy, he became instructor at the Lehranstalt (formerly Hochschule) für die Wissenschaft des Judentums, a rabbinical seminary in Berlin.
Shortly after the infamous Kristallnacht, Rosenthal left Germany in December 1938 and went to Sweden, where he was invited through the offices of the Swedish historian of religions H.S.
From there he went to England, where he arrived in April 1939, and eventually came to the United States in 1940, having received an invitation to join the faculty of the Hebrew Union College (HUC) in Cincinnati, Ohio.
His publications range from a monograph on Humor in Early Islam to a three-volume annotated translation of the Muqaddimah of Ibn Khaldun to a Grammar of Biblical Aramaic.