Shortly after her brother was killed in battle in World War I, Katharine McLennan decided she wanted to join other young women overseas as a nurse's aide in the military hospitals. Her father, however, would not hear of his youngest daughter tending to sick and dying soldiers. Frustrated by his refusal, Katharine enlisted the help of her sisters to convince him to let her go to France with the Red Cross Society, the Secours aux Blesses Militaries. J.S. eventually relented and she headed overseas in the spring of 1916.

Katharine's first commission was in Hospital de l'Alliance in Yvetot from May 10 to December 28, 1916. This was followed by Hospital Auxiliaire No.109 in Pont-Audemer from January 18 to September 18, 1917. From there it was on to Vasseny at a hospital referred to only as HOE 18; she was there from October 13, 1917 to February 28, 1918. Katharine took a break in the spring and early summer of 1918, but returned to France to Pontoise and the Hospital Militaire, Caserne de Cavaliere from July 16 to November 11, 1918. She also spent time in a German hospital in Langenschwalbach from January to May of 1919.

Her friends, Edith Parkman and Helen Homans, were her constant companions at work and her sightseeing partners on breaks. Her letters home are filled with descriptions of her daily activities, amusing stories, touching reminiscences of the wounded soldiers, and questions and comments about people back home. Katharine had developed an interest in photography and she kept scrapbooks of the haunting images she took of her experiences in France.

The brutality and cruelty of the war was disturbing to her however, and this comes through in the sketches she made of the soldiers in the hospitals. As the war dragged on, Katharine's correspondence expressed a tiredness with the situation and more specifically, with the lack of trained nurses.

Katharine returned from war wiser and more self-assured. Her nursing experience had transformed the shy, sheltered girl into a more confident and independent woman, who would continue to serve local Red Cross and Victorian Order of Nurses chapters in Cape Breton.