As a newly commissioned ensign, Donald Showers reported to Pearl Harbor in February 1942 and was assigned to the Combat Intelligence Unit, known as Station HYPO, under Commander J. Rochefort. He performed research and other support functions in exploiting the main Japanese naval code. In late 1944, he was selected as deputy to CAPT Edwin Layton, Admiral Nimitz's intelligence officer, to help establish a fleet combat intelligence center (Advance Intelligence Center) on Guam.

He developed innovative methods for SIGINT shadowing of Japanese fleet movements, and for distributing sanitized SIGINT for targeting.

In the postwar period, RADM Showers was instrumental in acquiring and maintaining order of battle information on adversaries of the United States. He served as fleet intelligence officer and as an instructor in intelligence schools.

During the Vietnam War, the Director of the Defense Intelligence Agency placed Rear Admiral Showers in charge of an effort to find and correct breaches of security that had given North Vietnamese forces knowledge of Strategic Air Command Operations. RADM Showers formed the PURPLE DRAGON program, an interagency group, comprising members from NSA, the staff of the Joint Chiefs of Staff, and Service Cryptologic Agencies. The program's findings resulted in greater combat effectiveness and saved American lives in Vietnam.

RADM Showers retired from the Navy in 1971 and continues to be held in high esteem among intelligence professionals. The "Rear Admiral Donald M. Showers Award," is presented to the Honor Graduate of the Naval Intelligence Officers Basic Course at the Navy Marine Corps Intelligence Training Center.