Ludlow served under Maj. Gen. Joseph Hooker in the Atlanta Campaign, and was appointed a brevet captain for gallantry at the Battle of Peachtree Creek.

After the Civil War, Ludlow devoted his life to a military career, serving in engineering and scientific capacities in the Dakota Territory, Washington, D.C., and in river and harbor management in Philadelphia, the Great Lakes, Nicaragua and New York City.

He briefly commanded the Department of the Visayas before being returned to the U.S. at the end of June 1901, due to the discovery of advancing pulmonary tuberculosis he had contracted in Cuba.

A story published in an 1884 issue of the "Engineering News and American Contract Journal"[6] featured Ludlow as an example of integrity in Army and civil service.

"That's a cool fellow," said an up-town manufacturer pointing across Chestnut Street, where Colonel [William] Ludlow, the Chief of the Water Department of Philadelphia, was waiting for a streetcar.

Then with a careless gesture Colonel Ludlow rolled up the $50 bill into a paper lighter, reached up to the gas, allowed it to become thoroughly ignited, and slowly lit his own cigar.

This done, the Colonel turned with an easy and polite motion, and said: 'Permit me,' and held the blazing bill under the nose and up to the cigar of my amazed and startled friend, whose eyes had now become almost as big as dinner plates.

My friend gets purple in the face every time he thinks of the affair, and confided it to me simply to warn me how to behave myself at the Water Department.