On May 19, 2020, about 1930 central daylight time, a Cessna 501, N501KM, sustained substantial damage when it was involved in an accident at Houston Hobby Airport (HOU), Houston, Texas. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. While on final approach to HOU, the pilot lowered the landing gear and received two of three green landing gear position indicator lights. The left main landing gear light was not illuminated, and a warning horn sounded. The pilot recycled the landing gear with the same result. He reported the situation to HOU tower and asked if they could visually see whether all the airplane’s landing gear were down. The tower reported that all gear appeared down, as did the pilot of a commercial airliner who was taxiing on the airport. As per the pilot operating handbook, the pilot initiated an aggressive bank right, followed abruptly by a left bank to generate sufficient lateral G-forces on the landing gear in an attempt to get a gear locked indication on the left main gear. This attempt did not work. Upon landing, left main landing gear collapsed, the airplane veered left and came to a stop in a grassy area adjacent to runway resulting in structural damage to the left wing and underbelly of the airplane. The pilot exited the airplane and airport emergency services responded. Recovery personnel manually lifted and locked the left main landing gear down, and the airplane was towed to a hangar facility. The pilot reported that he decided to land because the left main landing gear indicator light had malfunctioned in years past, and it was found that the light had malfunctioned rather than the landing gear. The landing gear was examined under the supervision of a Federal Aviation Administration (FAA) airworthiness inspector. Utilizing ground hydraulic and electrical power, the landing gear was cycled about 30 times with various hydraulic pressures. Both the nose landing gear and right main landing gear functioned normally. The cockpit landing gear position lights (nose, left main, and right main) functioned normally. The left landing gear actuator never locked in the down position under any hydraulic pressure setting. The only way to attain a locked down left main landing gear was to manually push the gear outward to the down and locked position. When the landing gear was pushed into the locked position, the left main landing gear down indicator light illuminated green. Evaluation of the left main landing gear actuator revealed that its internal down-lock mechanism was not functioning. According to the certified repair facility, the actuator is a component that is normally replaced at interval (37,000 cycles) or as needed. The FAA inspector reported that the aircraft logbook showed the left main actuator was original to the airplane and was last inspected on July 6, 2017, at 5,287 cycles. The logbook did not show any recent entries concerning anomalies with the left main landing gear.