Sidney (also spelled Sydney) Johnston Jennings, younger brother of J. Hennen Jennings*, was a son of James Rody Jennings and his wife Katherine Sharpe Hennen. The elder Jennings owned a coal mine in Hawesville, Kentucky, and had large shipping interests in New Orleans. Sidney received his schooling in Tours (France) and Hanover (Germany). Returning to the United States he qualified as a civil engineer at the Lawrence Scientific School at Harvard, Massachusetts, in 1885. He first worked as a surveyor and assayer for two years at the New Almaden Quicksilver Mine in California, where his brother Hennen was superintendent, and then as surveyor for the Anaconda Mines in Montana. In December 1889 the two brothers came to the Witwatersrand, where Sidney was appointed manager of the Willows Copper Argentiferous Syndicate, near Pretoria. Two years later he became assistant general manager of De Beers Consolidated Mines at Kimberley, and in 1893 manager of Crown Deep and Langlaagte Deep in Johannesburg. In August 1893 he married Amy Florence Horne and they eventually had two sons and two daughters. Around this time he achieved a record speed in driving tunnels which stood for many years. From 1896 to 1899 he was general manager of Crown Reef Gold Mining Company and during that period contributed to the doubling of its profits. At the same time he was consulting engineer to the Robinson Mine, and assistant consulting engineer to H. Eckstein and Company. He also served as an examiner in mining for the University of the Cape of Good Hope during 1898-1902.
In 1899, at the beginning of the Anglo-Boer War, Sidney went to England but returned the next year. He was appointed a member of the Johannesburg Town Council by the British military and became chairman of the council's works committee for the length of his stay in the Transvaal. He remained with H. Eckstein and Company as consulting engineer to 1907. Like his brother Hennen he served on the Technical Education Commission, appointed by the Lieutenant-governor of the Transvaal Colony in 1903. Its recommendations led to the establishement of the Transvaal Technical Institute. This institution became the Transvaal University College in 1906, with Jennings as chairman of its council. It gradually developed into the University of the Witwatersrand.
Sidney was a member of the American Institute of Mining Engineers and of the (British) Institution of Mining and Metallurgy. In February 1898 he joined the South African Association of Engineers and Architects, serving on its council during 1898/9. In the latter year it changed its name to the South African Association of Engineers and he was elected its president for 1906/7. When the South African Association for the Advancement of Science was founded in 1902 he was elected joint vice-president, a position he filled until he left South Africa. At the annual congress of the association at Kimberley in 1906 he was president of Section C and delivered his presidential address on "Wastes in mining". He also became a member of the Chemical and Metallurgical Society of South Africa (from 1902 the Chemical, Metallurgical and Minging Society of South Africa) in 1897, and in 1905 joined the British Association for the Advancement of Science.
In 1907 Jennings started a firm of consulting engineers in New York and the next year became vice-president and consulting engineer of the United States Smelting, Refining and Mining Company, a large coal-producing concern in Utah. As part of his work he established the model mining town of Hiawatha and provided social welfare services for the miners. He became one of the leading engineers in the United States, was elected president of the American Institute of Mining and Metallurgical Engineers for 1918-1919, and president of the American Mining Congress from 1922 to 1923.