He attended classes at the Chicago Art Institute in 1901 and in 1903 became assistant art editor at the Tribune. While attending the Francis Smith Art Academy of Chicago in 1900, he met fellow student Lillian Lusk, whom he marrieed in 1903. After moving to Battle Creek, Michigan, Koerner found a position as art editor, and illustrator for Pilgrim: A Magazine for the Home, which had a short life. In 1905, the Koerners moved to Detroit, where he became art editor for the United States Daily, a newspaper that folded after three months.

After another stint at the Tribune, Koerner and his wife made their way to New York City. At the Art Students League in 1905 – 06, Koerner studied with George B. Bridgman and received the Bridgman Scholarship in drawing. Wilson Dexter, a friend of Koerner from the League persuaded him to apply for instruction with Howard Pyle, and in 1907, Koerner was accepted as a student in Pyle’s weekly composition class. He moved to Wilmington and rented one of the studios maintained by Henry Downard. His neighbors there were Anton Otto Fischer, Macy Ellen Sigsbee Ker, and William H. Foster.

Upon the death of Pyle in 1911, Koerner wrote a tribute in honor of his teacher for publication in the New Amstel Magazine. Koerner exhibited in the First annual exhibition of paintings by the pupils of Howard Pyle held in Wilmington in 1912. The Koerners developed a close friendship with the wildlife illustrator Frank Stick and his wife Maud. With three other Pyle students, Percy V E Ivory, Herbert Moore, and E. Roscoe Schrader, the Koerners rented a house, “Naamans on Delaware,” where they lived for two years (1909-11). The Koerners’ next move was to 1502 Van Buren Street in Wilmington, and Koerner rented a studio at 1008 Franklin Street next to Anton 0. Fischer, for several years, before using the studio adjacent to Frank Schoonover’s at 1616 Rodney. A daughter, Ruth Ann, was born to the Koerners in 1913, and a son, named for his father, arrived in 1915.

In 1917, the Koerner family finally settled in Interlaken, New Jersey near Frank Stick and his family. The Koerners and the Sticks traveled to Montana in 1924, the first of many such trips for Koerner, who became well-known as an illustrator of cowboys, Indians, and the Wild West.

Koerner spent the last three years of his life as a bedridden invalid, unable to paint, He died, aged fifty eight, on August 11, 1938, His reconstructed studio became pact of the Buffalo Bill Historical Center at Cody, Wyoming, in 1978.