Sydney Twentyman Jones, judge, was the son of the merchant Thomas Jones and his wife Sarah Elizabeth Head, born Twentyman. He was born in London during a visit of his parents to the United Kingdom. He attended the Diocesan College at Rondebosch, Cape Town, and then studied at the South African College during 1866 and 1867, obtaining the second class certificate of the Board of Public Examiners of the Cape Colony in 1868. He proceeded to Trinity Hall, Cambridge, that same year, where he was awarded the degree Bachelor of Laws (LLB) in 1872 and was prizeman and scholar of the year. Three years later he obtained the degree Master of Laws (LLM). By that time he was back in South Africa, where he was admitted to the Cape Bar in February 1874. Owing to his painstaking work, mainly of a commercial nature, he developed a successful practice. In 1882 he was appointed as a judge of the Supreme Court and assigned to the High Court of Griqualand West as senior puisne judge. He was interested in both general and legal education and played a leading role in establishing the undenominational Kimberley Boys' High School. In July 1887 he was transferred to the Eastern Districts Court in Grahamstown. From time to time he acted as examiner in law for the University of the Cape of Good Hope and in 1890 the University of Cambridge conferred an honorary Doctor of Laws degree on him. He served as Judge President of the Eastern Districts Court from 1901 until he retired in July 1904. He retired to England, but returned to South Africa.
During the eighteen-nineties Justice Jones participated actively in some of Grahamstown's scientific societies, and showed an interest in prehistoric archaeology. In February 1891 he became a member of the committee of management of the Albany Museum. This committee was elected by members of the Grahamstown Literary, Scientific and Medical Society. He was elected joint vice-president of this society in July 1892, and member of a sub-committee which investigated its name and constitution. In July 1896 he was again elected joint vice-president of the society and of its museum committee, and re-elected the next year.
Meanwhile in May 1892 the defunct Eastern Province Literary and Scientific Society (1883-1886; 1892-1898?) was re-established and Justice Jones elected as its first president. In August 1893 he delivered a lecture before its members titled "Some evidence of primitive man", including a brief review of stone artefact finds in the Cape Colony during the previous 30 years and showing examples of artefacts both from his own collection and that of the Albany Museum. A summary of the lecture was published in the society's journal, the E.P. Magazine (Vol. 2(1), pp. 1-16). A report of the event in the Grahamstown Journal reflected scepticism with regard to the presumed great age of the artefacts. Jones was re-elected as president of the society several times, the last time in August 1895. In 1899 he presented an arrow straightener from Upington to the Albany Museum. It had eight grooves and was of a type hitherto unknown to the museum.
Jones was married to Florence Hayter Aderne in 1878. They had five sons and three daughters.