Kate (Katie) Gardiner was born on 21 September 1885 at Wavertree, Lancashire, England, to Frederick Gardiner, a shipowner, and his wife, Alice Evans. Her mountaineering career began early. As a child she accompanied her father on climbing trips in the European alps and was 10 years old when she climbed Mt Breithorn.
Her childhood among the mountains nurtured a lifelong interest and in 1925 she came upon a photograph of Mt Cook when sorting through her dead mother's possessions. This brought her out to New Zealand the following year. Guides at the Hermitage, Mt Cook, had expected a very experienced climber; her progress on her first climb was 'painfully slow'. Katie Gardiner was far from fit at the time, but her determination impressed the guides. Two years later, in 1928, she returned to New Zealand where she became the eighth woman to climb Mt Cook. She did some of her climbing with Englishman Harold Porter, who wrote of her patience in the face of jealousy from another woman climber and her equanimity when coping with difficult conditions.
Having finished a successful climbing season in New Zealand, Gardiner headed for the Canadian Rockies where she climbed Mt Assiniboine, the Matterhorn of this range, and made six first ascents with guide Walter Feuz. This set a pattern which was to continue for the next 10 years, as she spent the New Zealand summers climbing in the Southern Alps and the rest of the year climbing in Canada and Switzerland, becoming one of the foremost women climbers in the world. One of her most notable seasons in New Zealand was the summer of 1930–31, when she climbed 27 peaks, 9 of which were first ascents.
The following year Katie Gardiner began what was to become an epic struggle for the summit of New Zealand's second-highest peak, Mt Tasman, her goal since she climbed Mt Cook. During the 1932–33 season Katie and her guides made three unsuccessful attempts, each of which was foiled by bad weather. On the third occasion a sudden storm forced Katie, Englishman A. M. Binnie, and guides Vic Williams and Jack Pope to shelter in a crevasse, where they became stranded. On the sixth day Katie wrote a will to ensure that provision was made for her guides' wives. On the ninth day the weather cleared, they ate the last of their food and headed down the Fox Glacier, where they met a rescue party who had all but given them up for dead. Gardiner's survival was vindication for those struggling against the idea that mountaineering was too tough a sport for women.
Undeterred by her experience, Gardiner headed for the Rockies and more first ascents. She returned to New Zealand later in the year and finally achieved her goal of climbing Mt Tasman (the second woman to do so), as well as climbing another 20 peaks. She had contributed to the cost of a small hut on Pioneer Ridge in the Fox névé and this was to aid her Tasman expedition and those of countless others. Her next New Zealand expedition took her further south, where she climbed Mounts Aspiring and Tūtoko.
Katie Gardiner climbed in an era when men and women alike relied heavily on guides. A woman of means, she could afford to hire their services for the duration of her holidays and to finance her extensive travel. Reserved, with a keen sense of humour and great kindness, she was held in high regard by her guides, who named a hut in her honour at Mt Cook. She was described as a slow but steady climber with great stamina.
The Second World War interrupted Gardiner's climbing career, and she worked as commandant of an auxiliary convalescent unit in England. Although having no time for climbing, she served as the president of the Ladies' Alpine Club (1941–43).
In 1948 Gardiner returned to New Zealand to visit her friends Irene Chambers and Kate Small in Hawke's Bay, and climbed Mounts Egmont and Ruapehu. Two years later she came back to live permanently with Chambers, and later with Kate Small in Hastings. Her last climb, in 1951, was of the Minarets at the head of the Franz Josef Glacier, where 'she went as well as anyone half her age'.
Katie Gardiner had never married. She died in Hastings on 29 January 1974.