Answer: Neil Armstrong

Sixteen astronauts flew on ten manned Gemini missions through the early to mid-1960s. Among the sixteen there was only one civilian pilot. There were nine active-duty United States Air Force pilots, six active-duty United States Navy pilots, and one civilian pilot: Neil Armstrong.

Now, lest you think NASA was taking a chance on an untested civilian pilot, it’s worth noting that Armstrong was a civilian by retirement; prior to his life at NASA he was a decorated Navy pilot and then later accomplished test pilot involved in testing of such air craft as the Bell X-5, Thunderchief, Delta Dart, and was one of only eight pilots employed in the Parasev paraglider research project.