In 1829 he became editor of the Dorset County Chronicle. In 1840 Sydenham left the Dorset Chronicle and became editor of The West Kent Guardian, a Greenwich paper. In January 1846 he returned to Poole and started The Poole and Dorsetshire Herald, of which he was editor and part-proprietor.
Sydenham was the eldest son of John Sydenham, a bookseller of Poole, Dorset, born on 25 September 1807.
He was educated in his native town. He died at Poole on 1 December 1846.