[1] He served in the United States Army from 1861 to 1864, during the American Civil War.

He was confirmed by the United States Senate on May 28, 1864, and received his commission on June 20, 1864.

On March 3, 1871, Caldwell was reassigned by operation of law to serve only in the Eastern District.

[2] Caldwell was nominated by President Benjamin Harrison on February 27, 1890, to a seat on the United States Circuit Courts for the Eighth Circuit vacated by Judge David Josiah Brewer.

Caldwell was assigned by operation of law to additional and concurrent service on the United States Court of Appeals for the Eighth Circuit on June 16, 1891, to a new seat authorized by 26 Stat.