Mount Kirkby is a very large, prominent, linear, flat-topped mountain on the northern face of the Porthos Range in the Prince Charles Mountains of Antarctica. Situated about 3 nautical miles (6 km) east of Crohn Massif, it is approximately 9 kilometres (5 nmi) long east-west and 4 kilometres (2 nmi) wide at the western end, tapering to the east. It rises to 2,438 metres (8,000 ft) (460 metres (1,500 ft) above the plateau). The mountain was first visited by the Australian National Antarctic Research Expeditions southern party led by William G. Bewsher in December 1956, and was named by the Antarctic Names Committee of Australia for Sydney L.