The first underground station at King's Cross opened as part of the original section of the Metropolitan Railway in 1863 and was rearranged in 1868 and 1926. New platforms for the sub-surface lines of the Underground were opened about 400m to the west in 1941 to make interchanging between the sub-surface lines and the tube lines easier; the 1868 platforms later became the former King's Cross Thameslink station, which closed on 9 December 2007 when the Thameslink service moved to St Pancras International.
The Great Northern, Piccadilly and Brompton Railway (GNP&BR, now part of the Piccadilly line) platforms opened with the rest of the line in December 1906, while the City & South London Railway (C&SLR, now part of the Northern line) arrived in May 1907. The Victoria line platforms came into use on 1 December 1968 with the opening of the second phase of the line. The Victoria line escalators cut through the location of the original Piccadilly line lifts.
On 18 November 1987 the station was the scene of the devastating King's Cross fire. The cause was attributed to a lit match falling into, and setting fire to, an escalator machine room, combined with a then-unknown fire phenomenon of the trench effect, which caused the fire to explode into the station, killing 31 people. As a result, fire safety procedures on the Underground were tightened, staff training was improved and wooden steps on escalators were replaced with metal ones. The existing prohibition of smoking throughout the London Underground network was tightened. Due to the extensive damage caused by the fire, it took over a year to repair and reopen the station; the Northern line platforms and the escalators from the ticket hall to the Piccadilly line remained closed until 5 March 1989.
On 7 July 2005, as part of a co-ordinated bomb attack, an explosion in a Piccadilly line train travelling between King's Cross St Pancras and Russell Square resulted in the deaths of 26 people.