Neil Armstrong was the first man to walk on the Moon, but his space experience was much more than that. Neil was the commander of Gemini 8, the mission that gave the U.S. an edge by making the first docking in space (even though the mission had to be aborted soon after that docking). Before that Armstrong piloted the X-15 in the early 60’s.
But Armstrong was not a Mercury Astronaut. The original 7 Mercury Astronauts were Alan Shepard, Gus Grissom, John Glenn, Scott Carpenter, Wally Schirra, and Gordon Cooper (Deke Slayton was a Mercury Astronaut but never flew during Mercury because of a heart murmur. He did fly in space as part of the Apollo-Soyuz Test Project).