Born in Marylebone in London, the son of psychiatrist Forbes Benignus Winslow and Susan Winslow née Holt, Winslow was possibly the most controversial psychiatrist of his time. As a boy he was brought up in lunatic asylums owned by his father, and was educated at Rugby School and Gonville and Caius College, Cambridge, transferring to Downing College at the University of Cambridge after four terms, where he took the MB degree in 1870. He was also a DCL(1873) of Trinity College Oxford and LL.D. of Cambridge University. A keen cricketer, Winslow captained the Downing College XI. In July 1864 he was a member of the Marylebone Cricket Club (MCC) team which played against South Wales, in which team was W. G. Grace.
In 1871 he was appointed a Member of the Royal College of Physicians (MRCP). He spent his medical career in an attempt to persuade the courts that crime and alcoholism were the result of mental instability. His attempt in 1878 to have Mrs Georgina Weldon committed as a lunatic at the instigation of her estranged husband William Weldon resulted in one of the most notorious court cases of the nineteenth century. The public notoriety the Weldon case caused earned him the displeasure of the medical establishment, which continued even after his death.
He became an adherent of the benefits of hypnotism in dealing with psychiatric cases. He took an active role in securing a reprieve for the four people sentenced to death for the murder by starvation of Mrs. Staunton at Penge in 1877. In 1878 he inquired into the mental condition of the Rev. Mr. Dodwell, who had shot at Sir George Jessel, the Master of the Rolls. Other trials in which Winslow was involved were those of Percy Lefroy Mapleton, convicted of the murder on the Brighton line; that of Florence Maybrick, and that of Amelia Dyer, the Reading baby farmer. He also appeared in many civil actions.