Richardson was a philologist of the school of John Horne Tooke. In 1815 he published Illustrations to English Philology, consisting of a critical examination of Samuel Johnson's A Dictionary of the English Language and a reply to Dugald Stewart's criticism of Horne Tooke's Diversions of Purley. The book was reissued in 1826.
In 1818 the opening portions of an English lexicon, by Richardson, appeared in the Encyclopædia Metropolitana. In 1834 he issued the prospectus of a New English Dictionary, and the work itself was published by William Pickering in parts between January 1835 and the spring of 1837. It was published in volumes, with additional front matter and some omitted words including "retrospect" appended, in April 1837. The dictionary is a republication of the lexicon, with improvements and additions. Richardson's principle was to rely on etymology. He was severely criticised by Noah Webster in his Mistakes and Corrections (1837), especially for his ignorance of oriental languages. "Tooke's principle", wrote Webster, "that a word has one meaning, and one only, and that from this all usages must spring, is substantially correct; but he has, in most cases, failed to find that meaning, and you [Richardson] have rarely or never advanced a step beyond him". In quotations from authors the dictionary was far more copious than any previous work of its class in English. Despite many technical failings the work was commended by the Quarterly Magazine and the Gentleman's Magazine. An abridged single volume edition, without the quotations, appeared in 1839, with a new preface, but uncorrected.
He also published a book on the study of language, an explanation of Tooke's Diversions of Purley (1854). He contributed papers to the Gentleman's Magazine, and wrote essays on 'English Grammar and English Grammarians,' and on 'Fancy and Imagination.'
