He was encouraged to draw by his parents, and had local training in Baltimore and studied portraiture in Philadelphia from 1831 to 1832 with Thomas Sully. He studied in France from 1833 to 1834 and Italy at the English Life School in Rome. Returning to Baltimore, he opened a studio, but it was not successful.

In 1836, he established a studio in New Orleans where he met Captain William Drummond Stewart, a Scottish aristocrat and British Army officer, who engaged Miller to accompany him on a Rocky Mountain trip in 1837. The idea was for Miller to make sketches that he could later convert to oil paintings for Stewart's castle in Scotland.

The resulting sketches, about 200, in various media, and notebook studies of mountain men and Indians, mostly from Southwestern Wyoming, gave psychological insight into the subjects. These depictions captured the end of the heydey of the mountain men and also showed many scenes from Indian life. However, the works were not intended for public display but for the personal enjoyment of Stewart.

The sketches were shipped to Stewart's ancestral home, Murthly Castle in Scotland. Miller, fulfilling his commitment to Stewart, lived at the Castle from 1840 to 1842, and painted scenes in oil from their journey. This arrangement was made by Stewart after the death of his older brother from whom he inherited the castle.

Miller then settled in Baltimore, making a good living from oil paintings from numerous copies of his Rocky Mountain sketches and from portraiture. With many of his paintings, he supplied narrative descriptions, but, unlike many Easterners who traveled West before white settlement, he never published written descriptions of his western adventures.

Credit: Peggy and Harold Samuels, "Illustrated Biographical Encyclopedia of the American West", Michael David Zellman, "300 Years of American Art"