His parents were Just Ulrik Jerndorff (1806-1847), painter to the Oldenburg Court, and Nancy Caroline née Jones (1809-1877).

[4] In 1880, alongside Laurits Tuxen (1853–1927) and Frans Schwartz (1850–1917), he was a co-founder of a painting academy at Søkvæsthuset on Christianshavn in Copenhagen which served as a precursor of Kunstnernes Frie Studieskoler which were founded two years later.

[7] Jerndorff started his career as a landscape painter under the influence of Skovgaard, but soon turned to portrait painting and Biblical subjects.

He is best known for a number of monumental portraits which were painted for the Danish museum of National History at Frederiksborg Castle.

Together with Thorvald Bindesbøll, Otto Haslund, and Joakim Skovgaard, Jerndorff also worked with ceramics in the mid-1880s, a collaboration that led to the foundation of the Dekorationsforeningen society in 1887.