John Fletcher was apprenticed to a contractor and afterwards to the Borough Engineer of Dewsbury, Yorkshire, England, for three years. He was assistant to the Borough Engineer of Nottingham, before being appointed engineer to the Aston Rural Sanitary Authority near Birmingham. This was followed by an appointment as Borough Engineer of Sutton Coldfield, near Birmingham. In April 1889 he became Borough Engineer of Durban, a post held until his retirement in 1918 because of ill health. During this time he was consulted also by the municipalities of Johannesburg, Pretoria, and other South African towns. He was an associate member of the (British) Institution of Civil Engineers.
In January 1891 Fletcher produced an extensive report on the best method of sewage disposal, Report on the sewerage of the borough of Durban, Natal, which was published in Durban. He appears to have been a cultured person, citing reading and music as his hobbies. In October 1904 he was appointed a member of the Natal Technical Education Commission, which made recommendations relating to the development of tertiary and technical education in the colony. He was a founding member of the South African Association for the Advancement of Science in 1902, and served on its first council during 1902/3. He served on the committee for Section D (which included engineering and sanitary science) in 1903/4, and was joint vice-president of the association during 1903/4 and 1904/5. In 1907/8 he was a member of the association's local committee for Natal, and he was still a member of the association in 1910. In December 1911 he read a paper on "The newer conception of the municipality" before the Durban Sanitary Association. The paper was printed in a local newspaper and as a pamphlet.
John Fletcher's son, John Stanley Fletcher, was an engineer draughtsman in his father's office by 1906.