Johnson was appointed surgeon's mate in the navy, and sailed to
Newfoundland and
Nova Scotia, visiting the naval hospitals whenever his ship was in harbour. In January 1800 he passed his second examination, and in February he was made full surgeon and was appointed to the sloop-of-war . He accompanied the expedition against the
French forces in Egypt, but was forced to return to London invalided. He spent the winter in studying anatomy at the theatre in
Great Windmill Street School of Anatomy, and in June 1801 obtained an appointment on the sloop-of-war , and served in the
North Sea. At the
Peace of Amiens of 1802, Johnson was again out of work for a time; but in the following year (May) sailed for the East, and did not return to England till January 1806. In 1808 he was appointed to of 74 guns, in which he remained nearly five years, and saw active service. He attended the disastrous
expedition to Walcheren in 1809, and was there attacked with ague. At the
peace of 1814, Johnson served in , when the
Duke of Clarence conveyed the
Emperor of Russia and the
King of Prussia to the United Kingdom. He attended the Duke for a slight attack of fever, was appointed his surgeon
in ordinary, was on good terms with him, and, after the Duke's accession to the throne in 1830, became physician extraordinary. ==Peacetime physician==