After acting as amanuensis to the famous Samuel Parr, the vicar of Hatton in Warwickshire, he married and settled down at Thetford in Norfolk, where he lived for about twenty-five years. He was in the habit of adding the initials O.T.N. (of Thetford, Norfolk) to the title-page of his published works. In later life he became involved in a law-suit in connection with a will, and thus exhausted his means. In 1837—1838 he was a prisoner for debt in the king's bench and in the Fleet. He died in London on the 21st of March 1839. Barker was a prolific writer on classical and other subjects. In addition to contributing to the Classical Journal, he edited portions of several classical authors for the use of schools. He was one of the first commentators to write notes in English instead of Latin.
In a volume of letters he disputed the claims of Sir Philip Francis to the authorship of the Letters of Junius; his Parriana (1828) is a vast and illdigested compilation of literary anecdotes and criticisms. He also saw through the press the English edition of Lempriêre's Classical Dictionary (revised by Anthon) and of Webster's English Dictionary. It is as a lexicographer, however, that Barker is chiefly known.
While at Hatton, he conceived the design of a new edition of Stephanus's Thesaurus Graecae Linguae. The work was undertaken by AJ Valpy, and, although not expressly stated, it was understood that Barker was the responsible editor. When a few parts had appeared, it was severely criticized in the Quarterly Review (xxii., 1820) by Blomfield; the result was the curtailment of the original plan of the work and the omission of Barker’s name in connection with it. It was completed in twelve volumes (1816-1828). The strictures of the Quarterly were answered by Barker in his Aristarchus Anti-Blomfieldianus, which, although unconvincing, was in turn answered by Bishop Monk.
Continental scholars entertained a more favourable opinion of him than those of his own country. He expressed contempt for the minute verbal criticism of the Porsonian school, in which he was himself deficient.