He was one of twelve pilots who flew the North American X-15, an experimental spaceplane jointly operated by the U.S. Air Force and NASA.

He graduated with a Bachelor of Science degree in Aeronautical Engineering from the U.S. Air Force Institute of Technology in 1958.

In 1960, he was one of six test pilots selected to fly the X-20 Dyna-Soar, which was slated to become the first winged orbital space vehicle capable of lifting reentries and conventional landings.

After the X-20 program was canceled in 1963, he completed the astronaut training curriculum through the Aerospace Research Pilot School (Class 63A) at Edwards AFB and was selected to fly the North American X-15.

While climbing through 107,000 feet (33,000 m) at Mach 4.17 on June 29, 1967, he suffered a total electrical failure and all onboard systems shut down.

After reaching a maximum altitude of 173,000 feet (53,000 m), he calmly set up a visual approach and, resorting to old-fashioned "seat-of-the-pants" flying, he glided down to a safe emergency landing at Mud Lake, Nevada.

During 16 flights in the aircraft, Knight also became one of only five pilots to earn their Astronaut Wings by flying an airplane in space, reaching an altitude of 280,500 feet (85,500 m).

After nearly ten years of test flying at Edwards AFB, he went to Southeast Asia in 1968, where he completed a total of 253 combat missions in the F-100 during the Vietnam War.

Following his combat tour, he served as test director during development of the F-15 Eagle at Wright Patterson Air Force Base in Dayton, Ohio.