Hjerlow was born in Christiania (now Oslo). He received early training under Jacob Calmeyer, graduated from the National Academy of Craft and Art Industry in 1880, and then became a student of Knud Bergslien. He studied in Munich in 1882 and in Paris and Brittany from 1889 at 1891. He received the A. C. Houen Grant in 1890 and 1891. In 1883 he attended Frits Thaulow's "open-air academy" at Modum, and he debuted at the Autumn Exhibition in 1883. Hjerlow attended the 1889 World Exhibition in Paris, where he received honorable mention.