World War II interrupted his racing career but brought new flying opportunities. After jobs with Douglas Aircraft, Midcontinent Airlines, and General Motors, LeVier began a career with Lockheed Aircraft in April 1941. At first, he ferried Lockheed Hudson bombers built for the Royal Air Force, but he soon worked his way into the Engineering Flight Test Department and remained there for 32 years. He conducted the most extensive compressibility dive program that had been done and not only improved the P-38 Lightning, but also helped to pave the way for future high speed and supersonic flight. His skill in the P-38 led to an assignment as a special research test pilot in the Eighth and Ninth Fighter Command in England. To improve the combat effectiveness of the P-38, he conducted lectures, flight tests, and demonstration flights at all Eighth Air Force P-38 fighter bases. He then returned to Lockheed and helped America enter the jet age.
On 10 June 1944, LeVier took off from Muroc Field in the XP-80A, "Gray Ghost," the predecessor to America’s first production jet aircraft. He went on to make "first flights" in 19 other aircraft including the T-33, F-94, XF-104, and U-2. During his flying career, he flew more than 250 different aircraft, but his contributions to aviation were not limited to test flying. As an inventor, LeVier made many significant improvements in aircraft systems. He designed the master caution warning light system, the automatic wing stores release, the first practical afterburner ignition system, the "hot microphone" intercom system, and the placement of the trim switch on top of the control stick. After 10 years as Lockheed’s chief engineering test pilot, he became Lockheed’s Director of Flying Operations--a position he held until shortly before his retirement in 1974. He was a founding member of the Society of Experimental Test Pilots.