Sternen became acquainted with Impressionism already in Graz. In Vienna, he saw the original paintings of several French impressionists. In Munich he studied with fellow countrymen Rihard Jakopič and Matija Jama, two other representatives of Slovene impressionism. Unlike them, Sternen preferred figurative art, and his work consists mostly of portraits and female nudes.
He became known chiefly as a restorer and conservator of old paintings, and dedicated the majority of his later life to restoration. He partly restored and partly repainted the ceiling in the Franciscan church in Ljubljana, originally decorated by Matevž Langus in the mid 19th century, but badly damaged in the Ljubljana earthquake of 1895.
During World War II, Sternen sympathized with the pro-Nazi collaborationist policy of general Leon Rupnik and even painted a portrait of him. He nevertheless suffered no persecution after the end of the war. He died in Ljubljana on 28 June 1949 and was buried in the Žale cemetery.