In the spring of 1876, he headed north to the Black Hills with his camera, spending the summer making images among the mines.
In January 1877, he produced portraits of Brigadier General George Crook and of the court-martial board for Colonel Joseph J. Reynolds.
In the spring of 1877, he joined partnership with Joseph H. McGowan and traveled along the Union Pacific Railroad, taking portraits and selling his Black Hills views.
In the fall of 1877, he apparently visited the Red Cloud Agency where he took portraits of a majority of the Oglala and Arapaho headmen.
The largest surviving collection of Mitchell's Indian portraits from the Red Cloud Agency were preserved by Captain John G. Bourke, former aide-de-camp to Brigadier General George Crook.