Dorothy Michaelis Theomin was born in Dunedin on 24 December 1888, the second of two children of David Edward Theomin, a merchant, and his wife, Marie Michaelis. She was first educated at Braemar House, the Misses Miller's school in Dunedin. By then she had already gone abroad with her parents and would remain an enthusiastic traveller all her life.
The Theomin businesses prospered and at the start of the twentieth century David and Marie spent time in Europe planning a grand new house to be built on the site of their Royal Terrace residence. The London architect Ernest George was chosen to design it. Between 1902 and 1905 Dorothy attended Roedean School near Brighton in Sussex. It had been founded by the sisters Dorothy, Penelope and Millicent Lawrence, and its curriculum cut free from the stifling restraints of Victorian tradition. Girls were educated to be independent and self-reliant, and their physical and intellectual potential was emphasised. The challenging but supportive environment for girls of widely varying religious and cultural backgrounds was well-suited to the development of a Jewish girl, and its influence in forming her character was profound.
When the family finally returned to Dunedin, the newly completed Olveston was the talk of the town with its 35 rooms and a staff of five to seven servants. It shortly became the centre for much social activity stemming from business connections, family involvement with the arts and many philanthropic enterprises, and the commitment of David Theomin to the synagogue. After 1907 Marie became deeply involved with the newly formed Society for the Promotion of the Health of Women and Children (later the Plunket Society) at both regional and national levels. As a result she had little time left for domestic affairs, and Dorothy took over the running of Olveston. Her lifelong friend, Brenda Bell of Shag Valley station in North Otago, observed that under Dorothy's supervision Olveston 'ran on oiled wheels'.
Dorothy Theomin played golf and rode, but her great love was for the mountains of the Southern Alps. An asthmatic, she found conditions on the West Coast conducive to her health. She was a member of the New Zealand Alpine Club, and made an impressive number of climbs and transits between 1914 and 1933. In the early years she wore a short skirt, puttees and hob-nailed boots. The Graham brothers, Peter and Alexander, and Frank Milne were the guides who most frequently accompanied her. She was also a respected mountain photographer.
After 1933, as the sole survivor of her immediate family, Dorothy Theomin continued the philanthropic activities of her parents. From 1941 to 1955 she was on the executive of the Plunket Society, and throughout the 1939–45 war was active in the Red Cross, corresponding and sending food parcels to people during difficult times. She kept up the Theomin Trust for education and charitable purposes. Most important of all to her, however, was her interest in the arts. She had a good singing voice and played her own accompaniments on the fine Steinway piano in Olveston's drawing room. Her knowledgeable support and guidance encouraged young artists and musicians and was matched by her quiet financial backing. In the same way she was committed to the advancement of the Dunedin Public Art Gallery, becoming vice president of its council from 1954 to 1957 and president from 1957 to 1959.
On 11 October 1966 Dorothy Theomin died where she had been born, at Royal Terrace, leaving her house and its contents in trust for the citizens of Dunedin. She was buried in the family plot in the Jewish section of Dunedin's southern cemetery. She had never married, and worked to uphold her parents' belief that good fortune was on lease, to be employed for the benefit of others. She was forceful and dedicated, yet her drive was tempered by an infectious measure of fun, which became evident when she entertained guests, and a real love of living.