Mallory was born in Mobberley, Cheshire, the son of Herbert Leigh Mallory (1856–1943), a clergyman who changed his surname to Leigh-Mallory in 1914. George had two sisters and a younger brother Trafford Leigh-Mallory, the World War II Royal Air Force commander.
In 1896, Mallory attended Glen-gorse, a preparatory boarding school in Eastbourne on the south coast of England, having transferred from another preparatory school in West Kirby. At the age of 13, he won a mathematics scholarship to Winchester College. In his final year there, he was introduced to rock climbing and mountaineering by a master, R. L. G. Irving, who took a small number of people climbing in the Alps each year. In October 1905, Mallory entered Magdalene College, Cambridge to study history. There, he became good friends with members of the Bloomsbury Group including James Strachey, Lytton Strachey, Rupert Brooke, John Maynard Keynes, and Duncan Grant, who painted several portraits of Mallory. Mallory was a keen oarsman and rowed in the college eight for his three years at Cambridge.
After gaining his degree Mallory stayed in Cambridge for a year writing an essay he later published as Boswell the Biographer (1912). He lived briefly in France, where Simon Bussy painted his portrait, now in London's National Portrait Gallery. On his return he decided to become a teacher. In 1910 he began teaching at Charterhouse School, Godalming, Surrey, where he met the poet Robert Graves, then a pupil, and he went on to act as best man at Graves' wedding in 1918. In his autobiography,Goodbye to All That, Graves remembered Mallory fondly both for his encouragement of Graves' interest in literature and poetry and his instruction in climbing. Graves recalled: "He (Mallory) was wasted (as a teacher) at Charterhouse. He tried to treat his class in a friendly way, which puzzled and offended them."
While at Charterhouse he met his wife, Ruth Turner (6 October 1892-6 January 1942), who lived in Godalming, and they were married in 1914, just six days before Britain and Germany went to war. George and Ruth had two daughters and a son: Francis Clare (19 September 1915–2001), Beridge Ruth, known as 'Berry' (16 September 1917–1953), and John (born 1920). In December 1915 Mallory joined the Royal Garrison Artillery as 2nd lieutenant and in 1916 participated in the shelling of the Somme, under the command of Major Gwilym Lloyd George, the son of then Prime Minister David Lloyd George.
After the war he returned to Charterhouse, resigning in 1921 in order to join the first Everest expedition. In between expeditions he attempted to make a living from writing and lecturing, with only partial success. In 1923 he took a job as lecturer with the Cambridge University Extramural Studies Department. He was given temporary leave so that he could join the 1924 Everest attempt.1
On June 8, 1924, Mallory and his co-climber Andrew Irvine disappeared near the summit of Mt. Everest. It has never been established with certainty, whether the two reached the top of the mountain. Mallory's body was finally discovered by an expedition in 1999, while the body of Irvine is still missing.