Her father was Dr. James McIlroy, a medical practitioner in Ballycastle. She followed in her father's footsteps and attended Glasgow University in 1894 to do a medical degree.
She was one of the first women medical graduates, winning class prizes in both medicine and pathology before obtaining her MB,ChB in 1898 followed by an MD with commendation in 1900.After further postgraduate work throughout Europe specialising in gynaecology and obstetrics she was appointed gynaecological surgeon at the Victoria Infirmary in Glasgow, a post she held from 1906 to 1910.
At the outbreak of the First World War she and other female medical graduates offered their services to the government. They were declined as "the battlefield .. no place for women". They decided to set up the Scottish Women's Hospital for Foreign Service. Dr. Louise McIlroy commanded a unit of the hospital at Troyes, France before being posted to Serbia and, three years later, Salonika.
During her time in Salonika she established a nurses training school for Serbian girls and oversaw the establishment of the only orthopaedic centre in the Eastern Army. She finished her war service as a surgeon at a Royal Army Medical Corps hospital in Constantinople.