Victor Horsley was born in Kensington, London, and educated at Cranbrook School in Kent and at University College London, where he studied medicine under John Burdon Sanderson and George Dancer Thane. In 1880 he was appointed House Surgeon at University College Hospital where he experimented with anaesthetics. Horsley studied at postgraduate level in Berlin in 1881 and in 1882 was appointed Surgical Registrar at University College Hospital. From 1884 to 1890 Horsley was Professor-Superintendent of the Brown Institute, where he did experiments on localization of brain function (with Charles Beevor), on the pituitary gland, on the relation of the larynx to the nervous system (with Felix Semon), and on the thyroid gland, myxoedema and cachexia strumipriva. In 1885 he was promoted to assistant surgeon. In 1886 he took the position of Assistant Professor of Surgery at the National Hospital for Paralysis and Epilepsy, Queen Square, where he performed operations on the brain and spinal cord. In 1886 he was appointed secretary of the Local Government Board Commission on Hydrophobia, and also studied Pasteur's anti-rabies vaccine. In the same year he was elected Fellow of the Royal Society. From 1887 to 1896 Horsley was Professor of Pathology at University College London. He married Eldred, daughter of Sir Frederick Bramwell, in 1887, and the couple had two sons and one daughter. Horsley was elected President of the Medical Defence Union in 1893 and the British Medical Temperance Association in 1896. In 1897 he was appointed to the Senate of the University of London and elected to the General Medical Council. From 1899 to 1902 he was Professor of Clinical Surgery at University College London. In 1902 he was knighted for his work in medicine. In 1907 he published 'Alcohol and the Human Body' with Dr Mary Sturge. Towards the end of his life he stood as a Liberal candidate in London but later resigned; he was also rejected as a candidate for Leicester.
Horsley was commissioned in the Territorial Army in 1910 as captain in the 3rd London General Hospital of the Royal Army Medical Corps. On the outbreak of the First World War, he reported for active duty on the Western Front where he was initially sent as surgeon at the British hospital at Wimereux, France. He was next posted in May 1915 as a colonel and Director of Surgery of the British Army Medical Service in Egypt, based at the 21st General Hospital in Alexandria, in support of the Dardanelles Campaign. In the following year he volunteered for field surgery duty in Mesopotamia, where he died unexpectedly in Amarah, Iraq, on 16 July 1916, of heatstroke and severe hyperpyrexia, at only 59 years of age.
Lady Horsley continued to be involved in radical causes after her husband's death. Their sons, Siward and Oswald, were both educated at Bedales School in Hampshire, then at Oxford University. Both fought in the Great War, the younger, Oswald, being killed in a flying accident at the end of 1918. The elder, Siward, died in 1920. In 1917 Victor's daughter Pamela married Stanley Robinson, who was knighted in 1972 for his work in the British Museum. Pamela and her husband helped to found a Babies Club in Chelsea.