A relative of Richard Brinsley Sheridan, Knowles enjoyed success writing plays for the leading West End theatres.

The family moved to London in 1793, and at the age of fourteen Knowles published a ballad entitled The Welsh Harper, which, set to music, was very popular.

[1] Although Dr Willan offered him a share in his practice, Knowles decided to give up medicine for the stage, making his first appearance as an actor probably at Bath, and played Hamlet at Crow Street Theatre, Dublin.

[1] His first important success was Caius Gracchus, produced at the Belfast Theatre in 1815; and his Virginius, written for William Charles Macready, was first performed in 1820 at Covent Garden.

He published two polemical works: the Rock of Rome and the Idol Demolished by Its Own Priests in both of which he combated the special doctrines of the Roman Catholic Church.