Mary Victoria Gibson was the eldest of 10 children of Frederick Denhame Gibson and his second wife, Mary Fox Rodd. She was born on 28 October 1864 at Lyttelton, New Zealand, where her father was port officer. Her sister, Helena Fannie (known as Helen) was born at Lyttelton on 14 July 1868. All of their six sisters and two brothers were to have some involvement with Rangi-ruru (later Rangi Ruru Girls' School), which Helen Gibson founded in 1889. Mary Gibson was for 30 years principal of Christchurch Girls' High School. A third sister, Beatrice, became principal of Nelson College for Girls.
Mary Rodd had met Frederick Gibson on the ship of which he was captain, on a voyage from England to Australia in 1863. She was a forthright woman who saw to it that her daughters had an education equal to that of her sons. Both Helen and Mary attended Mrs Crosby's school in Christchurch and then Christchurch Girls' High School, which had been founded in 1877 to prepare girls to enter Canterbury College. When the family moved to Dunedin for two years in 1884, Mary, Helen and their sister Beatrice remained in Christchurch to continue their education. Mary graduated BA from Canterbury College in 1887 and MA in 1888. Helen attended lectures at the college and its School of Art, and continued to paint throughout her life, but unlike her sister she did not graduate. Neither sister married.
Helen Gibson began teaching privately while at university. In 1889 her father bought out a Papanui school run by friends of the family, sisters Ada, Kate and Jessie Gresham. Helen borrowed a horse and cart to visit the parents of the existing pupils, nearly all of whom agreed to remain at the school under the new management. At first only her younger sister, Alice, and her mother helped her run Miss Gibson's Private School for Girls. In 1891, when the school moved to Webb Street, it was named Rangi-ruru (wide sky-shelter) by Paora Taki of Rapaki, a friend of Captain Gibson.
With only a year's absence, when she visited England in 1906, Helen Gibson presided over Rangi-ruru for almost 50 years. The school prospered under her leadership. Her sisters Alice, Ruth, Ethel and Winifred all taught there for many years, and the school developed in line with the family's strict Christian moral code and lively sense of humour. From 18 pupils in 1889 there were over 200 at the time of Helen Gibson's death in 1938. In a prize-giving speech to parents in 1933 she summarised her aim: 'to build up character – to hand back to you, girls who will prove strong in body, pure in mind, and cultured in intellect.' Helen Gibson found public speaking difficult and lacked confidence generally, but would always force herself to perform such tasks, if possible asking her sister Beatrice to write her speech for her.
While Helen was leading Rangi-ruru, Mary Gibson took charge of Christchurch Girls' High School in 1898. Previously she had been headmistress of the girls' department of East Christchurch School since 1889, having begun her teaching career at Sydenham School while still at university. As principal of Christchurch Girls' High School she presided over a period of continual expansion, including the establishment of the boarding house, Acland, and a separate branch of the school which in 1927 became Avonside Girls' High School. Like her sister, she immersed herself in the life of her school. She strongly encouraged sporting and social activities, and regularly played the piano at school parties. An expanding school roll – from 126 in 1898 to over 600 by 1926 – did not prevent her from continuing to take a personal interest in her pupils and their subsequent careers. In later years she regretted a diminishing interest which she perceived among the girls in pursuing professional careers.
The tributes paid to Mary Gibson on her retirement in 1928 were fulsome. The next year she took up a position as relieving principal at Waitaki Girls' High School in Oamaru. While there she fell ill with pneumonia and died on 1 September 1929. Helen Gibson never officially retired from teaching but gradually withdrew as age restricted her movements. She remained principal of Rangi-ruru until her death at St George's Hospital, Christchurch, on 24 July 1938. She is commemorated in a stained glass window in St Mary's Church, Merivale, where there are also two brass plates in memory of her mother, Mary Fox Gibson. Together with their sisters, Mary and Helen Gibson had directed the education and development of thousands of girls in Canterbury over five decades.