He settled in Chicago. One of his first jobs was as a decorator of façades for the Riverview Amusement Park in Chicago. In 1918, he formed his own decorating company, John A. Mallin, Interior Art Decorations. He specialized in church decoration and ecclesiastical painting as well as art glass and mosaics. He stated his philosophy in one of his commercial brochures: "As there is nothing too good for God, so there is nothing too rich or too precious for God's earthly homes, His churches..."