As a young child he was educated by a tutor at Wentworth Woodhouse, his uncle Thomas's family seat in South Yorkshire.

Later he was sent to Caen in Normandy, where a Calvinist academy or university existed at that time and where Wentworth is supposed to have been taught by Samuel Bochart.

In 1649 he had succeeded to the Earldom of Roscommon, which had been created in 1622 for his great-grandfather, James Dillon; and he was now put in possession by an act of the Irish Parliament of all the lands owned by his family before the Civil War.

[15] The essay contained the first definite enunciation of the principles of poetic diction, which were to be fully developed in the reign of Queen Anne.

Alexander Pope, who seems to have learnt something from his carefully balanced phrases and the regular cadence of his verse, says that "In all Charles's days, Roscommon only boasts unspotted bays";[16] in his An Essay on Criticism, when Pope lists poets he admires, beginning from the classical age, Roscommon is one of two British poets he includes (William Walsh is the other).

Roscommon believed that a low code of morals was necessarily followed by a corresponding degradation in literature, and he insists that sincerity and sympathy with the subject in hand are essential qualities in the poet.

Roscommon formed a small literary society that he hoped to develop into an academy with authority to formulate rules on language and style, but its influence only extended to a limited circle, although it included such men of distinction as John Dryden and George Savile, 1st Marquess of Halifax, the scheme fell through after its promoter's death.

His family became extinct in 1816, and in 1828 Michael James Robert Dillon, another descendant of the 1st Earl, established his right to the earldom before the House of Lords.