Ismael Gentz ​​was a son of the painter Wilhelm Gentz, from whom he also received his training. His earliest surviving works include sketches that he made on a trip to Karlovy Vary and that brought him a complaint with the police for harassment. In the late 1870s he traveled with his father to Algiers via Italy. Portraits he drew on this trip were later recorded in words and pictures in Georg Ebers' work Egypt. From 1879 he attended the art academy in Berlin. In 1888 and 1889 he studied at the Düsseldorf Art Academy. Peter Janssen the elder was his teacher there. In the years 1889/90 he took his parents on a trip to Tunis and Tripoli. From the later 1880s he regularly took part in the Great Berlin Art Exhibition. In 1892 he stayed in Paris to study. He was best known for his portraits of contemporaries. The portraits were partly lithographically, partly also photomechanically reproduced.

Ismael Gentz ​​died in Berlin in 1914 at the age of 52 and, like his father before, was buried in the Old St. Matthew Cemetery in Schöneberg. Both graves have not been preserved.