Dillon is the eldest of five children of Sir James Charles Mathew and Elizabeth Blackmore Mathew. Her family is related to the Butler family, but she does not visit Ireland until 1886. Living in Queen’s Gate Gardens, Kensington, London, she is educated at home. From a young age she attends the ladies’ gallery of the House of Commons, while mixing a busy social life with charitable works.
Dillon begins keeping a diary in 1879, which she continues to write until her death. Her ancestor, Mary Mathew, is also a diarist and keeps the diary for the discipline of the daily activity. She soon begins to write for the love of it, and some have surmised she wrote with the intention her diaries would be read by others. She attends lectures in Old English and literature at King’s College, London from late 1882 to 1884, and begins to learn Irish in 1893.
Dillon’s father supports land reform in Ireland, chairs the evicted tenants commission in 1892, and is a huge influence on her politics. She makes her first political reference on February 25, 1883 when she notes the arrest of the Invincibles, and she then regularly comments on land reform. She travels to Ireland for the first time in August 1886, staying in Killiney, County Dublin. In October 1886, she meets John Dillon, and begins to follow the Plan of Campaign so that she can discuss it with him during his visits to the Mathew house in London.
During this time, John Dillon is deeply immersed in politics, and is imprisoned on a number of occasions. Being a careful follower of Irish politics, she becomes an anti-Parnellite. She confronts John Dillon in autumn 1895 about their relationship, saying that they can no longer meet as they had become the subject of gossip. He proposes within two weeks, and they are married on November 21, 1895 in Brompton Oratory. They are busy and often apart, with Dillon spending time in a warm climate due to his ill health. She tries to accompany him when she can, but the couple’s large family makes that difficult. They have one daughter and five sons, John Dillon (1896-1970), Anne Elizabeth Dillon (born October 29, 1897), Theobald Wolfe Tone (1898-1946), Myles, James, and Brian.
Finances are strained until John’s uncle Charles bequeaths him his house, 2 North Great George’s Street, Dublin in 1898, and a business in Ballaghaderreen, County Mayo is bequeathed him by a cousin, Anne Deane, in 1905. Dillon runs the business successfully, while also carrying out duties as a politician’s wife such as opening the Belfast ladies’ branch of the United Irish League in June 1905. Her busy life results in her neglecting her diary.
Dillon dies on May 14, 1907 in Dublin, having given birth to a stillborn daughter that morning. Pneumonia is given as the cause of death, but it could have been medical incompetence. She is buried in the family vault in Glasnevin Cemetery. Her husband writes of her death in June 1907, A short narrative of the illness and death of my dearest love. Trinity College Dublin holds her diary and correspondence. Her diaries, edited by Brendan Ó Cathaoir, are published in 2019.