The son of a wealthy leather merchant, Sladen was born near Halifax, Yorkshire on 30 June 1849.
He was educated at Hipperholme Grammar School and Marlborough College, but received no university training.
1877 also saw the publication of his first paper, in which he split the sea-lily genus Poteriocrinus into four; in his lifetime, Sladen would gain a reputation as a "splitter" because of his proclivity for declaring specimens to belong to new genera or species.
From December 1878, Sladen spent three months at Naples under the auspices of the British Association for the Advancement of Science.
In 1895 he was elected vice-president of the Linnean Society, but only a few months later he gave up both this and his secretarial position because of health problems.