At the age of 16, Tennant started his working life as a bookkeeper with the firm of Messrs Charles Bragg & Co. at Newcastle upon Tyne, and two years later was appointed to the staff of the Brandling Junction Railway on Tyneside, where he soon became Chief Clerk. in 1848 he became General Manager of the Leeds and Thirsk Railway and then, after working as Accountant for the newly formed NER, he took up the position of General Manager in 1870. He held this position until his retirement in 1891. After this, in the same year, he was elected Director of NER.