David Brown joined the U.S. Navy after his internship at the Medical University of South Carolina. Upon completion of flight surgeon training in 1984, he reported to the Navy Branch Hospital in Adak, Alaska, as Director of Medical Services. He was then assigned to Carrier Air Wing Fifteen which deployed aboard the aircraft carrier USS Carl Vinson in the Western Pacific. In 1988, he became the only flight surgeon in a ten-year period to be chosen for pilot training. He was ultimately designated a Naval Aviator in 1990 at NAS Chase Field in Beeville, Texas, ranking number one in his class. Brown was then sent for training and carrier qualification in the A-6E Intruder. In 1991, he reported to the Naval Strike Warfare Center at NAS Fallon, Nevada, where he served as a Strike Leader Attack Training Syllabus Instructor and a Contingency Cell Planning Officer. Additionally, he was qualified in the F/A-18 Hornet and deployed from Japan in 1992 aboard USS Independence flying the A-6E with VA-115. In 1995, he reported to the U.S. Naval Test Pilot School at NAS Patuxent River, Maryland as their flight surgeon where he also flew the T-38 Talon. David Brown logged over 2,700 flight hours with 1,700 in high performance military aircraft. He was qualified as first pilot in NASA T-38 aircraft. He held a Federal Communications Commission (FCC) issued Technician Class amateur radio license with the call sign KC5ZTC.

Selected by NASA in April 1996, Brown reported to the Johnson Space Center in August 1996. He completed two years of training and evaluation, and was qualified for flight assignment as a mission specialist. He was initially assigned to support payload development for the International Space Station, followed by the astronaut support team responsible for orbiter cockpit setup, crew strap-in, and landing recovery. On April 21, 2001, Brown appeared on ESPN as an expert on g-force loading on the human body that led to the cancellation of the Firestone Firehawk 600 CART race. David Brown flew aboard Space Shuttle Columbia on STS-107, logging 15 days, 22 hours, and 20 minutes in space from January 16 to February 1, 2003. The flight was a dedicated science and research mission. Working 24 hours a day, in two alternating shifts, the crew successfully conducted approximately 80 experiments. The mission ended abruptly on February 1, 2003, when Columbia’s crew perished during entry, 16 minutes before the scheduled landing.