After leaving college, Johnson served in
King's College Hospital as house physician and then house surgeon. In 1850 he was made an honorary fellow of King's College. He became an assistant physician at the hospital in 1847 and physician in 1856. From 1857 to 1863, he was Professor of
Materia medica (pharmacology) and from 1863 to 1876 he was Professor of Medicine. In 1862, Johnson was elected a member of the senate of the
University of London. In 1865 he was appointed a consulting physician to the British Home and Hospital for Incurables, replacing
Benjamin Guy Babington, who had resigned. In 1872, he was made a fellow of the
Royal Society. In 1876, Johnson attempted to treat
Charles Bravo, a British lawyer who was fatally poisoned with
antimony in what became known as "the Murder at the Priory".
The Lancet of August 1876 published his detailed account of the symptoms, treatment and progress of the illness. From 1876 to 1886, Johnson was Professor of Clinical Medicine at Kings. In 1883, he was appointed consulting physician to the
Royal College of Music. He was a Censor at the
Royal College of Physicians, and in 1887 was appointed Vice President of this institution. He was President of the
Royal Medical and Chirurgical Society in 1884. In 1889
Queen Victoria made him a Physician-Extraordinary. He was knighted in 1892. In 1850, Johnson married Charlotte Elizabeth, daughter of Lieutenant William White of
Addington, Surrey. They had five children.
Frank Holl painted his portrait in 1888. This picture is now held by the Royal College of Physicians of London. He died from cerebral hemorrhage at his home in
Savile Row, London, on 3 June 1896. A memorial to him lies in
St James's Church, Piccadilly. ==Work==