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.

Charles Philips Trevelyan (1870-1958) was the first son of George Otto Trevelyan and Caroline Lady Trevelyan of Wallington Hall, Northumberland. Educated at Harrow and Trinity College Cambridge, Charles' political career began with a role as Private Secretary to Lord Houghton at Dublin Castle. However he did not enjoy the work and returned to England in 1893. After his return to England, he ran an unsuccessful campaign as Liberal candidate for North Lambeth. While standing as Liberal candidate for Elland in North Yorkshire, Charles travelled through North America, the Pacific Islands, Australia and New Zealand with Beatrice and Sidney Webb. On his return to England in 1899 he won a by-election, becoming the representative of Elland. In 1908 he was also made Parliamentary Secretary to the Board of Education.
In 1904 Charles married Mary Katharine Trevelyan nee Bell (1881-1966) [Molly], daughter of Hugh and Florence Bell, and half-sister of the archaeologist and explorer Gertrude Bell. The pair had seven children together, six of whom survived to adulthood. The early years of their marriage were split between Cambo House on the Wallington Estate in Northumberland, and 14 Great College Street in Westminster.
When Britain declared war with Germany in 1914, Charles resigned his position in the government in protest – a move which attracted much criticism from his peers, the press and the public. Alongside others who disagreed with the conflict, Charles helped to establish the Union of Democratic Control (UDC) – a political group who publicly criticized the activities of the British government during World War I and advocated a peaceful resolution. As an active member, Charles promoted the UDC and its activities extensively, publishing articles and pamphlets, and giving talks around the country.
During his involvement with the UDC, Charles came into contact with many figures of the growing Labour Party. In 1918 Charles announced that his political sympathies were more aligned with Labour than the Liberals. Following Armistice with Germany that same year, Charles ran as a member of the Independent Labour Party for Elland. He lost his constituency of 18 years to a Conservative candidate, as part of the landslide Conservative coalition victory.
In 1921 Charles articulated his transition between parties in a short publication entitled From Liberalism to Labour. The following year he stood as Labour candidate for Newcastle Central and won. In the first Labour government of 1924, Charles was made President of the Board of Education, however his impact was limited by the short duration the government served. When Labour were re-elected to government in 1929, Charles was given the same role. However, he resigned after becoming frustrated with the government's reluctance to pass radical policy, in particular his bill to raise the school leaving age. Later that year Charles lost his seat at Newcastle to a Conservative candidate.
Following these disappointments Charles announced his retirement from politics. His parents' recent deaths had resulted in the family moving into Wallington Hall and taking on the management of the large estate. This gave Charles the opportunity to put into practice some of his socialist ideals, including the introduction of child benefit payments to tenant families and the establishment of pensions. In 1936 Charles announced he would bequeath the Wallington Estate to the National Trust. This became reality on his death in 1958.
Charles' wife Mary supported Charles' political career throughout her life, assisting with campaigning, hosting parties for political figures and in later years contributing to the running of the Wallington Estate. Early in their marriage she served as Chairman of the Northumberland Women's Liberal Foundation and she later became a Justice of the Peace. She also served on committees for the Campaign for the Protection of Rural England, the National Trust, the National Federation of Women's Institute and the Association of Country Women of the World. She was awarded the OBE in 1963.