John Sinclair Hedges joined the civil service of Natal Colony in November 1895 and was appointed deputy commissioner of mines at Nkandla in March 1896. After a spell as clerk and postmaster from October that year, also at Nkandla, he was re-appointed in his previous post on 1 May 1898. The full name of his post was deputy commissioner of mines and inspector of mines for the Eastern Mines Department District (Nkandhla). He served under the commissioner of mines, C.J. Gray*. In 1907 he was transferred from Nkandla to the office of the commissioner of mines in Pietermaritzburg. After the formation of the Union of South Africa (1910) he was appointed claims inspector under the deputy commissioner of mines for Natal in the Department of Mines and Industries in 1911. That same year he became deputy commissioner of mines for Zululand and south-western Natal.
Hedges was one of the first to examine the crystalline rocks intruded by granitoids in northern KwaZulu-Natal, now forming part of the Natal Structural and Metamorphic Province. This was in 1888, years before his appointment to the civil sercive. His observations were published in the annual reports of the commissioner of mines of Natal between 1888 and 1909. Furthermore, the geological map of Natal published in 1910 acknowledges him for recognising the Tugela fault.
Hedges was married to Kathleen S. Wallace.