In 1963, Walker made two X-15 flights beyond 100 kilometers - the edge of space. These were the only powered spaceplane flights past that threshold until SpaceShipOne in 2004. These flights qualified him as an astronaut under both U.S. Air Force and Fédération Aéronautique Internationale rules. Joe Walker was the first person to enter space twice.
After World War II, Walker joined the National Advisory Committee for Aeronautics' Aircraft Engine Research Laboratory in Cleveland, Ohio, as a physicist. While at the Cleveland facility, Walker became a research pilot and conducted icing research in flight as well as in the icing wind tunnel. He transferred to the High-Speed Flight Research Station in Edwards, California in 1951, to become a research pilot.
For fifteen years, Walker served as a pilot at the Edwards Flight Research Facility (now known as Dryden Flight Research Center); by the mid-1950s, he was a Chief Research Pilot. Walker worked on several pioneering research projects. He flew three versions of the Bell X-1: the X-1#2 (2 flights, first on August 27, 1951), X-1A (1 flight), X-1E (21 flights). When Walker attempted a second flight in the X-1A on 8 August 1955, the aircraft was damaged in an explosion just before being launched from the JTB-29A mothership. Walker, unhurt, climbed back into the mothership, and the X-1A was jettisoned.
Other research planes that he flew were the Douglas D-558-I Skystreak #3 (14 flights), Douglas D-558-II Skyrocket #2 (3 flights), D-558-II #3 (2 flights), Douglas X-3 (20 flights), Northrop X-4 Bantam (2 flights), and Bell X-5 (78 flights). For the X-3 Stiletto, Walker was project pilot and made all of the flights; he considered the X-3 the worst plane he ever flew. In addition to research aircraft, he flew chase planes and programs involving the F-100 Super Sabre, F-101 Voodoo, F-102 Delta Dagger, F-104 Starfighter, and the B-47 Stratojet.
In 1960, he was the first NASA pilot to fly the X-15 aircraft, following Scott Crossfield, the manufacturer's test pilot. On the first flight, he didn't realize how much power the engine had, and he crashed backward into his seat, screaming, "Oh, my God!"; a flight controller jokingly replied "Yes? You called?" Walker would go on to fly the X-15 24 times, including the only two flights to exceed 100 kilometers in altitude, Flight 90 (19 July, 1963; 106 km) and Flight 91 (22 August, 1963; 108 km). Walker was the first civilian to fly into space. He also achieved the fastest speed in the X-15A-1: 4,104 mph (Mach 5.92) during a flight on 27 June, 1962 (the fastest flight in any X-15 was Mach 6.7 by Pete Knight).
Walker was also the first pilot of the Bell Lunar Landing Research Vehicle, which was used to develop piloting and operational techniques for moon landings. On October 30, 1964, he took the LLRV on its maiden flight, reaching an altitude of 10 feet and a total flight time of just under one minute. He piloted 35 flights in all on the LLRV.
Walker's X-1E was decorated with nose art of two dice and the name "Little Joe" (Little Joe being a slang term in craps). Similar artwork — reading "Little Joe the II" — was applied to his X-15 for record-setting Flight 91. These were two rare cases of research aircraft carrying nose art.
Walker flew weather recon flights during WWII earning the Distinguished Flying cross, awarded by General Nathan Twining in July 1944 and the Air Medal with seven oak leaf clusters.
Walker was a charter member and one of the first Fellows of the Society of Experimental Test Pilots. He received the Robert J. Collier Trophy, the Harmon International Trophy for Aviators, the Iven C. Kincheloe Award and the Octave Chanute Award. His alma mater awarded him an honorary Doctor of Aeronautical Sciences degree in 1961. The National Pilots Association named him 1963 Pilot of the Year.
Joe Walker was inducted into the Aerospace Walk of Honor in 1991, and the International Space Hall of Fame in 1995. Joe Walker Middle School in Quartz Hill, California is named in his honor as well as the Joe Walker Elementary School in Lagonda, Pennsylvania.