Following her graduation in 1899, Lynn went to the United States, where she worked for ten years, before returning to Ireland to become the first female doctor at the Royal Victoria Eye and Ear Hospital (1910–1916).

Later on in her young life, in 1886 Lynn and her family moved to Cong, a village bordering Mayo and Galway, to where her father's parish was being funded by the Lady Ardilaun of Ashford Castle.

[6] Growing up in the aftermath of The Great Famine (1845 – 1852), She was deeply saddened by the deadly diseases and poverty suffered by the people in her local area.

[13] Lynn was a member of the radical British Women's Social and Political Union (WSPU) from 1908 and she was also said to be on friendly terms with the suffragist Sylvia Pankhurst.

"[20] She was given a gold fibula bone-shaped brooch as a token of gratitude from the Irish Citizen Army for her help in the medical preparation for the Rising.

[21] Lynn eventually left politics in 1927, increasingly frustrated by Sinn Féin's refusal to embrace social reform and health care.

Lynn became politically active and worked in Liberty Hall providing food and care for the poor and destitute families during the turbulent time of the 1913 Dublin Lock-Out.

[23] Lynn's medical career was defined by her work at Saint Ultan's Children's Hospital, which she established in Dublin in 1919, with a group of female activists.

Lynn's work with Dublin's inner-city poor had convinced her of the need for a hospital to provide medical and educational facilities for impoverished mothers and infants.

At the request of rebel leader James Connolly she joined the Irish Citizen's Army during the 1916 rising and was appointed as Captain and Chief Medical Officer.

[26] Lynn died on 14 September 1955, and was buried in the family plot at Deansgrange Cemetery with full military honours in recognition of her role in the Rising and the War of Independence.

The detailed diaries chronicle her medical, political and social life and were donated to the Royal College of Physicians in Ireland archive by her family in 1997,[28] which also holds the administrative papers of Saint Ultan's Hospital.

They provided source material for the 2010 documentary entitled Kathleen Lynn – An Dochtúir Reabhlóideach by Loop Line Film and director Sé Merry Doyle.

[29] The Diaries of Kathleen Lynn: A Life Revealed through Personal Writing edited by Mary McAuliffe and Harriet Wheelock were published in October 2023.

[10] Historian Margaret Ó hÓgartaigh wrote her biography, Kathleen Lynn, Irishwoman, Patriot, Doctor (Irish Academic Press, 2006).