George Edward Mannering was born on 31 July 1862 at Birch Hill station in North Canterbury, New Zealand. He was the son of Theophilus Samuel Mannering, a sheep farmer, and his wife, Annie Buckham, who ran a girls' school from the family home. George, or Guy as he was often called, was educated at Christ's College, Christchurch, and at various English schools during a family visit in 1876–77. He left school to work on the family farm at the age of 15, but farming did not suit him: he objected to the dirty work and hated sheep. In May 1878 he joined the Rangiora branch of the Union Bank of Australia. Mannering stayed with the bank throughout his working life, rising from 'stamp-licker' to manager of branches in Hastings, New Plymouth, Timaru, Napier and Christchurch.
Inspired by the Irishman W. S. Green's attempt in 1882 to climb Mt Cook, Mannering determined that the Southern Alps should not be left to 'foreigners and to visitors from distant lands'. With little experience and no alpine equipment, he teamed up in March 1886 with an English cousin, C. D. Fox, in the first attempt on Mt Cook by a New Zealander. The pair failed, but Mannering, undeterred, learnt all he could about climbing from alpine journals, correspondence with Green and hard winter experience in the Canterbury mountains.
From 1886 to 1896 he spent most summers exploring the Mt Cook region with Marmaduke Dixon, A. P. Harper and other Canterbury enthusiasts. Mannering made a number of successful climbs, including the first crossing of the Ball Pass and exploration of the Murchison Glacier with Harper in January 1890. He attempted Mt Cook five more times, getting to within 200 feet of the summit in 1890, but bad luck, changeable weather and lack of time frustrated each attempt. The summit was finally reached in 1894 by a group of New Zealanders led by T. C. Fyfe. Mannering led the first ascent of the low peak of Mt Rolleston in December 1891, the first crossing of Sealy Pass in February 1892 and the first ascent of Phipps Peak in December 1896. In 1897 he left Christchurch. His most active period of climbing was over, although he made what was probably the first winter ascent of Mt Egmont in 1903 and the first crossing of the Rutherford Pass with T. C. Fyfe in 1908. At the age of 60 he scaled many peaks in the Swiss Alps, including the Matterhorn, and he climbed Mt Torlesse in Canterbury to celebrate his 70th and 79th birthdays.
During the 1890s Mannering helped establish a recognisably New Zealand alpine climbing tradition. In his book With axe and rope in the New Zealand alps (1891) and in articles for newspapers and alpine journals, he attempted to foster an interest in the new sport among sceptical New Zealanders. He emphasised the challenge and satisfaction to be gained from direct contact with the unexplored natural environment, without the assistance from guides and porters which European climbers were used to. In 1891 Mannering, Harper and others formed the New Zealand Alpine Club to encourage mountaineering. Mannering served the club as first editor of the New Zealand Alpine Journal, as a member of the committee (1893–96), vice president (1914–31) and president (1932–34). In 1935 he was made an honorary life member of the New Zealand Alpine Club and the Alpine Club in London.
Mannering's early adventures extended beyond climbing. In 1889 he and Dixon made the first canoe descent of the Waimakariri River from Bealey to Kaiapoi, to the amazement of onlookers. The following year the pair canoed from Birch Hill Creek, near Mt Cook, to the mouth of the Waitaki River, a feat not repeated for over 70 years. Mannering was also an accomplished alpine photographer, an early racing cyclist, a fisherman, singer and competitive golfer. He wrote and provided most of the photographs for two books: Mount Cook and its surrounding glaciers (1930) and The Franz Joseph Glacier, New Zealand (1931). In 1943 he wrote an autobiography, Eighty years in New Zealand.
On 22 December 1894 in Christchurch Mannering married Lucy Harvey Lean. She enjoyed tennis, music and climbing and the couple shared a happy life together; they had two children. Mannering was shattered when in 1913, despite his attempts to rescue her, his wife drowned in the Aratiatia Rapids of the Waikato River. After living with his daughter Mildred for some years Mannering married a much younger woman, Dorothy Margaret Samuel, at Christchurch on 19 December 1921; the couple had three children. Dorothy Mannering died at Christchurch in 1942.
George Mannering was a rather reserved man who could appear severe on first acquaintance, but he was able to get on with a wide variety of people. He had a keen appreciation of nature and a willingness to try any mountain or river. Mannering was capable and persistent and probably ranks as New Zealand's first real alpine climber. He died on 29 October 1947 at his home in Fendalton, Christchurch.