Born New York, NY 1896, Pupil of his father Kenyon Cox; he attended the National Academy of Design and studied under George Bridgman. Awarded the American Academy Fellowship in Rome during 1916.
The son of noted artists Kenyon and Louise Howland Cox, Allyn was raised in Cornish, New Hampshire, and as a mature artist became one of the nation's leading mural painters.
In 1916, when he was age 20, he completed his first mural, which was installed over the mantle of the fireplace of the Windsor, Vermont Public Library. This occurred at the time he won a scholarship to study for three years at the American Academy in Rome, Italy.
Prior to winning the scholarship, he had studied in New York City at the National Academy of Design and the Art Students League. When he returned to New York, he received commissions for murals and decorative paintings for private homes, churches and public buildings and did murals for Dumbarton Oaks in Washington, DC and for Grant's Tomb and Memorial in New York. In 1952, he was commissioned to complete the last thirty-two feet of the frieze in the great rotunda of the United States capitol building, a project he had dreamed of completing from the time he had seen the blank area when he was a child visitor to the Capitol.
For this project which was completed in 1954, Cox used scenes from the Civil War, Spanish American War and the birth of aviation with the Wright brothers. He also did ceiling painting of the House of Representatives and in 1973, completed 16 panel murals for the House side of the Capitol Building.
In 1980, he had a serious heart attack and reduced his work load, and the Capitol Historical Society assisted him by installing a motorized chair to haul him to and from his scaffolding. But he retired in 1981, having devoted 30 years to mural painting in the nation's capitol. He died on September 26, 1982, and a week earlier, Senate Majority Leader Howard Baker, Jr. had said in a public talk: "Your splendid work enjoys a special place in the art history of the world."