Sydney was born in Wolverhampton on 31 August 1888, the son of John and Emma Jenkins. In 1901 he was living with his parents at 22 Lea Road, Wolverhampton, with sisters Emma J., Winifred, and Gladys and brother John W. He attended the Higher Grade School in Wolverhampton. By 1911 he is a boarder in Byfleet, Surrey, and has become an aeronautical engineer.
In 1912 he married Evelyn May Horton in Wolverhampton. However, from 1917, he appears to have had further bigamous marriages and children, for which he eventually went to court. Further details of this aspect of his life are discussed here.
Sydney enlisted in the Royal Flying Corps on 26 June 1912 (service number 112) and was in the 1AM 5 Squadron at the outbreak of war. He was promoted to Corporal on 10 August 1914 and proceeded to France. On 9 November 1914, he was awarded the French Medaille Militaire for gallantry. The Higher Grade School log book proudly proclaimed on 20 November 1914:
This morning after prayers read of the decoration of Sergeant Sidney Jenkins, formerly a pupil here, by the French President with the Medaille Militaire, for gallantry; announcement received with acclamation by scholars.
Wrote to Sergeant Jenkins conveying him our hearty congratulations.