NASA's requirements for astronaut groups 1 and 2 included test pilot experience. Group 3 didn't require candidates to be test pilots, but they did require a significant number of flight hours in high performance jet aircraft. Meeting those requirements was essentially impossible for anyone who had not been in the military.

Several NASA astronauts were not in the military at the time they were chosen though. Neil Armstrong had been a naval aviator, but he was working as a civilian research pilot for NASA when he was chosen as an astronaut. Elliot See worked as a research pilot for GE after having flown for the Navy.

Groups 4 and 6 (the Apollo-era scientist-astronaut groups) had astronauts who didn't have previous military service. Jack Schmidt (walked on the moon on Apollo 17) is the most famous example. Ed Gibson (science pilot on Skylab 4) also wasn't a veteran.