Who was the first person to make an untethered spacewalk?

Bruce McCandless II (June 8, 1937 – December 21, 2017) was a U.S. naval officer and aviator, electrical engineer, and NASA astronaut. In 1984, during the first of his two Space Shuttle missions, he made the first untethered free flight by using the Manned Maneuvering Unit (MMU).

"Challenger" launched from Kennedy Space Center, Florida, on February 3, 1984. The flight deployed two communications satellites, and flight-tested rendezvous sensors and computer programs for the first time. This mission marked the first checkout of the MMU and Manipulator Foot Restraint (MFR). McCandless made the first untethered free flight on each of the two MMUs carried on board, thereby becoming the first person to make an untethered spacewalk.

He described his experience:

"I was grossly over-trained. I was just anxious to get out there and fly. I felt very comfortable. It got so cold my teeth were chattering and I was shivering, but that was a very minor thing. I'd been told of the quiet vacuum you experience in space, but with three radio links saying, 'How's your oxygen holding out?', 'Stay away from the engines!' and 'When's my turn?', it wasn't that peaceful. It was a wonderful feeling, a mix of personal elation and professional pride: it had taken many years to get to that point."