Soper was nominated by President
Warren G. Harding on February 10, 1923, to a seat on the
United States District Court for the District of Maryland vacated by Judge
John C. Rose. He was confirmed by the
United States Senate on February 24, 1923, and received his commission the same day. His service terminated on May 9, 1931, due to his elevation to the Fourth Circuit. Soper served on the board of trustees of
Morgan State University for more than three decades (as its chairman for half that time), and helped bring it within the Maryland state university system. His last judicial act (as a senior judge) was an order allowing an African American, Henry Gantt, to attend the school of architecture at
Clemson University. ==Death and papers==