Mary Katharine Bell was born in Kirkleatham on 12 October 1881, to Sir Thomas Hugh Bell, second baronet and iron master, and his second wife, Lady Florence Eveleen Eleanor Bell (née Olliffe), author and social investigator. She was the youngest of their three children, and was the stepsister of the traveller, writer and political figure Gertrude Bell, and the granddaughter of Sir Isaac Lowthian Bell, first baronet, steel manufacturer and MP. Mary, known as Molly to close friends and family, studied briefly at Queen's College before she began a relationship with Liberal MP Sir Charles Philips Trevelyan, third baronet (1870-1958). After their marriage in January 1904, Mary became a successful political hostess at their home in London, arranging dinners and parties for political friends and associates. Around this time, Mary became president of the Northumberland Women's Liberal Federation (WLF), and became a popular speaker around the country. She favoured women's suffrage, an issue which deeply divided the WLF, but disliked the militancy associated with the movement. Mary actively aided her husband in his work, translating German material on land reform, and campaigning in his favour at each election, including that of 1918 in which he was attacked for his opposition of the First World War.

After Charles inherited the Trevelyan ancestral home of Wallington Hall, Northumberland, in 1928, the pair devoted much of their lives to the estate and the village of Cambo, becoming invested in the welfare of their tenants and establishing a pension plan for local schoolchildren. Mary was active in many local, national and international groups including the Workers' Educational Association and the Federation of Women's Institutes, as well as founding local branches of the Girl Guides, the Women's Institute and the Band of Hope. She served on the national executive of the National Federation of Women's Institutes, played a leading role in the Folk Dance and Northumbrian Pipers societies, and played a key role in the establishment of the Council for the Preservation of Rural England in 1926, becoming a representative and founding committee member. In 1963, she was awarded with an OBE for her years of public service to politics and various good causes. Mary died at St Catherine's Nursing Home in Newcastle Upon Tyne on 8 October 1966, and her ashes were scattered on moorland near Winter's Gibbet on the Wallington estate.

In addition to estate work she was a JP and was active with numerous local, national, and international organizations, such as the Workers' Educational Association and the Association of Country Women of the World. She founded branches of the Girl Guides and the Women's Institute in Cambo, and served on the national executive of the National Federation of Women's Institutes for many years. In keeping with her temperance principles she also founded in Cambo a Band of Hope, which local children were pledged to join at a young age. She also played a leading role in the Folk Dance and Northumbrian Pipers societies, and made music an important feature of her family's life. Singing songs, accompanied by Molly on the piano, was a regular part of their domestic routine, and she and Charles also delighted their children, among them George Lowthian Trevelyan, by regularly reading to them from a diverse selection of classic literature. Such activities took the place of attending church on Sundays, for while Molly was Unitarian, Charles was agnostic. In appearance her clothes, hairstyle, and pince-nez gave her a Victorian air, and one contemporary described her as attractive in a 'no nonsense' sort of way. Her decades of public service, to politics and various other good causes, were recognized with an OBE in 1963, shortly before her death at St Catherine's Nursing Home, Newcastle upon Tyne, on 8 October 1966. Her ashes were scattered on moorland near Winter's Gibbet on the Wallington estate.