Kathleen Lynn was born on the 28th of January, 1874 in Mullaghfarry, in the civil parish of Killala, Co. Mayo. Her parents were Robert Lynn, a Church of Ireland rector and member of the Protestant Ascendency, and Catherine Wynne, a descendent of the Earl of Hazelwood. Katherine was the second of four children, and her parents' high social status and wealth meant that she received a full education. When she was about 13 years old, Kathleen was sent to the Alexandra boarding school in Dublin, a prestigious Church of Ireland establishment. Although she had enjoyed a privileged upbringing, Kathleen bore witness to extreme poverty in rural Ireland throughout her childhood. This early exposure to the suffering of others instilled Kathleen with a great sense of empathy and a compassion for the poor. She decided to become a doctor so that she could be of some help to the less fortunate people of Ireland.
Kathleen attended the Catholic University Medical School at Cecilia Street, graduating in 1899. She also studied in Manchester and Düsseldorf. After graduating, Kathleen travelled to the United States where she built up her medical experience before returning to Ireland in 1909. Upon her return to Dublin, Kathleen became a fellow of the Royal College of Surgeons, but she found that many male colleagues refused to work with a woman. She moved from hospital to hospital, often as the first female doctor to work there, and eventually set up her own practice in her home in Rathmines.
In 1913, Kathleen became a supporter of the Dublin Lock Out and spent a large amount of her time volunteering in the Liberty Hall soup kitchen with Countess Markievicz. During this time Kathleen became sympathetic to the Irish Nationalist cause while she was treating Helena Molony with whom she would have long talks about politics and equality. She became involved in a number of political movements, including Irish Nationalism and Women's Suffrage. She became a member of the Irish Citizen Army and served as Chief Medical Officer in the 1916 Easter Rising, for which she was imprisoned at Kilmainham Gaol. After her release, she joined Sin Féin and was elected to Dáil Éireann in 1923 but retired from political activity in 1927.
Kathleen never married, but lived with fellow nationalist and activist Madeleine Ffrench-Mullen. The two shared a close relationship, and an analysis of Kathleen's diaries, which can be found on the Royal College of Physicians website, has suggested that the two were likely in a romantic relationship.
Dr Kathleen Lynn died in Dublin on the 14th of September 1955. She was buried in Deansgrange Cemetery and given full military honours in recognition of her contribution to the Nationalist cause.