Patrick (also Peter) Fletcher senior, son of Patrick Fletcher and his wife Agnes, born Campbell, should not be confused with his son, Patrick junior* - both were surveyors. Patrick senior claimed to have practiced as an architect and civil engineer in Glasgow, Scotland, from 1845 to 1850, but this period must have included at least some professional training. He then came to the Cape Colony and from March 1851 to January 1853 worked as an assistant in the government school at Stellenbosch. In October 1853 he passed the examinations set by the Colonial Engineer's Department and the next year passed the government survey examination. After a few months as temporary draughtsman in the Surveyor-General's office early in 1854 he spent three years surveying the lower Orange River and mineral leases in Namaqualand. From November 1857 he did survey work for the Cape Town and Wellington raiway line for three months, followed by thirteen years of surveying in Namaqualand. Among others he surveyed the Olifants River (1858-1859), mapped the roads to Clanwilliam (1860), laid out the towns of Springbok (1862) and Hondeklip Bay (1863), mapped the Hondeklip Bay to Springbok road (1863) and the proposed railway between Riethuis and Hondeklip Bay (1865). During 1863-1865 he kept a diary, which is now in the Cape Town Archives Repository. From about 1861 to 1871 he and his family lived on the farm Keerom (not identified).
In 1867 Fletcher was transferred from colonial surveyor to inspector of roads in charge of a section of the Hondeklip Bay road (known as the Messelpad). In June 1871 he became inspector of roads in the Eastern Cape. In 1885 he was appointed district inspector of public works, stationed in Port Elizabeth, and in January 1887 inspector and special justice of the peace for the Millwood gold fields near Knysna.
Fletcher wrote that he was intimately acquainted with nearly every part of the colony, from the mouth of the Orange River to near its headwaters and to Port St. Johns. He had given attention to its climate, geology and water resources over a period of 30 years and was convinced that irrigation could be developed almost everywhere in the territory. This was in 1885, when he wrote a contribution titled "Weir-system of irrigation, with special regard to the rivers of the colony" in The South African Exhibition, Port Elizabeth, 1885 (pp. 217-223), edited by C. Cowen. His knowledge of the geology of the colony is supported by his donation of a valuable collection of fossils and minerals, both local and foreign, to the Albany Museum, Grahamstown, in 1886. The next year he served on the Millwood Gold Commission and was co-author of two reports to Parliament: Report of the Millwood (Knysna) Gold Commission (1887), by S. Giddy, P. Fletcher and T.C.J. Bain*; and Report on the Knysna Goldfields, and on gold-mining and prospecting in the Colony, for the year 1887, by T.C.J. Bain, P. Fletcher and P.D. Hahn*. As Inspector of Mines at Knysna he submitted annual reports for the years 1888 to at least 1891. In August 1891 his appointment was changed to inspector and registrar of gold mines at Millwood, a post he filled to 1895. In that year he wrote a "Report on the discovery of coal near Cala, Mtimhlone outcrops", describing the coal seams discovered in the Transkei. The report was published in the Agricultural Journal of the Cape of Good Hope (Vol. 8(4), pp. 99-100).
In January 1860 Fletcher married Agnes Eaglesim, with whom he had seven sons and two daughters.