Jan (or Johannes) Gerhardus Philip Becker was the son of Jan Gerhardus Philip Becker, a farmer, and his wife Margaret Louise, born Steyn. There is some confusion about his first name. On his death certificate and in one of his obituaries he is named Jan, but in another obituary, in the records of the University of Glasgow and in other sources he is named Johannes. He received his school education at Riebeek-Wes and at the Boys' High School, Stellenbosch, where he matriculated (through the University of the Cape of Good Hope) in 1906. Proceeding to the University of Glasgow to study medicine he qualified as Bachelor of Medicine (MB) in 1912. After gaining some hospital experience in England he continued his studies at the University of Liverpool and obtained the Diploma in Public Health (DPH) and the Diploma in Tropical Medicine (DTM) in 1913.
Upon his return to South Africa Becker accepted an appointment in 1914 as pathologist and medico-legalist (later senior pathologist) at the South African Institute for Medical Research in Johannesburg. Except for a short break as a general practitioner in Standerton early on, he remained at the institute for the rest of his life. At first he was involved with general pathology, but later his research was mainly in bacteriology and parasitology. His publications included "A preliminary note on the intermediate host of Bilharzia haematobium in the Transvaal together with a description of the cercariae with which the mollusc is infected" (1916) and "A further note on bilharziasis in the Transvaal" (1916), both in the Medical Journal of South Africa, in which he proved by experiments with guinea pigs that the water snail Physopsis africana is an intermediate host of the bilharzia parasite.
During his last two years at the institute, after reaching retirement age (60) in 1947, Becker was mostly involved in medico-legal work and remained active to the day of his death. In 1920, when a Medical School had been established at the University College, Johannesburg (from 1921 the University of the Witwatersrand) he was appointed as the first part-time senior lecturer in bacteriology. In 1940 he was promoted to the first professor of bacteriology and parasitology.
Becker was highly respected and well-liked at the institute and university, not only for his extensive knowledge and the quality of his research, but also as a result of his humanity and helpfulness. He was a well-balanced, patient and retiring person and had a large number of friends. In his spare time he was a successful farmer, enjoyed hunting and fishing, and collected paintings and antiquities. He married Lea ("Lintie") Magdalena Theron, with whom he had a son and a daughter.