William A. Soga was a son of Reverend Tiyo Soga, a Xhosa, and his Scots wife Janet Burnside. One of his brothers was the veterinarian Jotello F. Soga*. Like his three brothers William was educated in Scotland under the protection of the United Presbyterian Church of Scotland. He studied divinity and medicine at the Universities of Edinburgh and Glasgow, qualifying at Glagow in 1883 as Bachelor of Medicine (MB) and Master of Surgery (CM). Two years later he was ordained as a minister of the United Presbyterian Church. That same year he married a Scots lady.
Returning to South Africa around 1889 as a medical missionary Soga was licensed to practice medicine in the Cape Colony on 30 May 1890. He was stationed in Bomvanaland (a region of the Transkei near Elliotdale), first at the mission station "Malan", and later in charge of the new mission station "Miller". His work was both medical and spiritual, but as there were no other doctors nearby his medical practice stretched over a large region and included both indigenous people and Europeans. His excessive work load led to a breakdown in 1891, but after his recovery he again took up his duties. The degree Doctor of Medicine (MD) was conferred upon him by the University of Glasgow in 1894. In 1904 he resigned his ministerial duties and moved to Elliotdale where he devoted himself to his large medical practice. He was appointed justice of the peace at Elliotdale in 1909.
Meanwhile in 1897 he used a fungus culture prepared at the Colonial Bacteriological Institute by Dr Robert S. Black* to successfully combat a locust plague near Elliotdale. He reported the work in a paper, "Locust-destroying fungus", published in the Agricultural Journal of the Cape of Good Hope (1897, Vol. 10(4), pp. 210-213).
Later he revised a pamphlet, "How to prevent consumption", originally published by William Hay. The revision was published in Cape Town, probably in 1905, by the "Association for the prevention of Consumption and other forms of tuberculosis in the Colony of the Cape of Good Hope".
In May 1913 his son, who had qualified in Glasgow, joined Soga in his medical practice. The resulting reduction in his work load led him to apply to the United Presbyterian Church to be reinstated in his ministerial capacity. After his appointment as an honorary missionary he frequently travelled on horseback over rough country in the line of duty. In 1914 he had a severe illness, with cardiac complications. Two years later he died unexpectedly while playing a round of golf. He was a widely esteemed and exceptionally intellectual person, remarkably well read not only in medicine and theology, but generally.