By 1812 he was an honorary surgeon at the Dispensary, Castle Hill, Lancaster that provided free medical treatment to the poor.
[2] In 1815 he organised the establishment of a local Board of Health that resulted in the foundation of Lancaster's House of Recovery for five patients, especially those with infectious diseases.
[2] Johnson was also one of the local surgeons who performed public dissections of those executed in Lancaster.
[3] As well as medicine, he was interested in science, especially microscopy, and disseminated his knowledge within the local community.
[2] He was a member of the Lancaster Medical Book Club (1823) and Leeds Mechanic Institute (1824).