Born at Cork he became a contributor to Blackwood's Magazine, and after moving to London in 1824 became for a few months in 1826 the Paris correspondent to The Representative, a paper started by John Murray, the publisher.

When its short career was run, he helped to found in 1827 the ultra Tory Standard, a newspaper that he edited along with a fellow graduate of Trinity College, Dublin, Stanley Lees Giffard; he also wrote for the more scandalous Sunday paper, The Age.

In 1837, Bentley's Miscellany was launched, with Charles Dickens as editor, and Maginn wrote the prologue and contributed over the next several years a series of "Shakespeare Papers" that examined characters in counter-intuitive fashion (e.g., the key to Falstaff is his melancholy).

From "The Man in the Bell" (Blackwood's, 1821) through "Welch Rabbits" (Bentley's, 1842) he was an occasional though skilful writer of short fiction and tales.

He also wrote The Military Sketch-Book: Reminiscences of Seventeen Years Service Abroad and At Home, "By An Officer of the Line" (anonymously) published in two volumes by Henry Colburn in 1827.