[1] During the First World War, DeWitt served as a captain in the United States Army Coast Artillery Corps.

[1] During the Second World War, he served as a major in the Monuments, Fine Arts, and Archives program, safeguarding historical buildings in the French countryside.

[4] He designed the Sam Rayburn Library and Museum in Bonham, Texas, in the Classical Revival architectural style in 1955.

[5] In the latter part of his career, DeWitt helped restore the original Senate and Supreme Court buildings and the James Madison Memorial Building of the Library of Congress in Washington, D.C.[1] He also helped design the Dallas neighborhood of Wynnewood, under the direction of businessman Angus G.

[6] Together with Mark Lemmon, DeWitt designed the Sunset High School, the Woodrow Wilson High School, some buildings on the campus of Southern Methodist University, and the Highland Park United Methodist Church in Dallas.