James Craib was the son of William Craib and his wife Ann, born Morrison. He qualified as Master of Arts (MA) at the University of Aberdeen. On the basis of his Scottish qualification he was admitted to the MA degree of the University of the Cape of Good Hope in 1889. Subsequently he was professor of mathematics and science at Gill College, Somerset East, until early in 1904. From 1901 or earlier to 1903 he was also honorary curator of the Gill College Museum and the college laboratories, though he did no research there during these years. The college provided some university education up to 1903, but then lost its university college status to become a high school only. Craib lived in Somerset East from 1893 or earlier, for in that year he was the town's meteorological observer for the Cape of Good Hope Meteorological Commission. From 1897 he managed a second order meteorological station for the Commission until 1904, when the observations were taken over by H.G.W. Adam, who also succeeded Craib as honorary curator of the Gill College Museum and laboratories. On 1 April of that year Craib was appointed Inspector of Schools in the Department of Education of the Cape Colony (from 1910 the Cape Province of the Union of South Africa), a post he still held in 1914.
In 1926 Craib was back in Somerset East and had presumably just retired. During that year he collaborated with his son-in-law, Basil F.J. Schonland, on important lightning research in South Africa. (For earlier investigations of atmospheric electricity see W.A.D. Rudge*). Schonland set up an intricate apparatus on Craib's farm Gardiol, near Somerset East, to measure the electric fields of thunderstorms and Craib became an enthusiastic participant in the work. Their observations proved conclusively that the base of most thunderclouds carries a negative electric charge, which supported the theory of the Englishman C.T.R. Wilson that thunderstorms are bipolar, with a positively charged region located above a negatively charged region. They measured the mean moment of the charge brought to ground during lightning discharges, and established that the fair weather electric field never exceeded 60 volts per meter. Their results were written up in an important paper, "The electrical fields of South African thunderstorms", with Craib as co-author, and published in the Proceedings of the Royal Society of London (1927, Vol. 114A, pp. 229-243).
James Craib was married to Isabella Cornelia Hofmeyr, with whom he had three sons and two daughters. Their daughter Isabella Marian ("Ismay") Craib was married to Schonland, while their eldest son, Dr. William Hofmeyr Craib, became a prominent cardiologist. James Craib should not be confused with David Craib (MA), who also served as honorary curator of the Gill College Museum and the college laboratories, from early in 1881 to at least 1884. In 1888 he too was an observer in Somerset East for the Cape of Good Hope Meteorological Commission. He joined the Cape civil service in May 1900 and, like James, was an Inspector of Schools from 1 January 1901 to at least 1910. In 1906, and still in 1910, he was a member of the South African Association for the Advancement of Science. He may have been an older brother of James.