He was born Michael O'Brien in County Cork, and was very young when the family moved to Limerick, Ireland. He attended the University of Dublin, and is believed to have been at one time a soldier in the British Army. On leaving college he went to London, and in the course of four years spent his inheritance of £8,000, meanwhile editing a periodical in aid of the World's Fair of 1851. About 1852 he came to the United States, in the process changing his name to Fitz James and thenceforth he devoted his attention to literature.
While he was in college he had shown an aptitude for writing verse, and two of his poems—"Loch Ine" and "Irish Castles"—were published in The Ballads of Ireland (1856).
His earliest writings in the United States were contributed to The Lantern, which was then edited by John Brougham. Subsequently he wrote for The Home Journal, The New York Times, and The American Whig Review. His first important literary connection was with Harper's Magazine, and beginning in February 1853, with The Two Skulls, he contributed more than sixty articles in prose and verse to that periodical. He likewise wrote for the New York Saturday Press, Putnam's Magazine, Vanity Fair, and The Atlantic Monthly. To the latter he sent "The Diamond Lens" (1858) and "The Wondersmith" (1859), which are unsurpassed as creations of the imagination, and are unique among short magazine stories. "The Diamond Lens" is probably his most famous short story, and tells the story of a scientist who invents a powerful microscope discovers a beautiful female in a microscopic world inside a drop of water. "The Wondersmith" is an early predecessor of robot rebellion, where toys possessed by evil spirits are transformed into living automatons who turns against their creators. His 1858 short called "Horrors Unknown" has been referred to as "the single most striking example of surrealistic fiction to pre-date Alice in Wonderland" (Sam Moskowitz, 1971). "What Was It? A Mystery" (1859) is one of the earliest known examples of invisibility in fiction.
His pen was also employed in writing plays. For James W. Wallack he made A Gentleman from Ireland, that held the boards for a generation. He also wrote and adapted other pieces for the theatres, but they had a shorter existence.
In New York he at once associated with the brilliant set of Bohemians of that day, among whom he was ranked as the most able. At the weekly dinners that were given by John Brougham, or at the nightly suppers at Pfaff's on Broadway, he was the soul of the entertainment.
In 1861 he joined the 7th regiment of the New York National Guard, hoping to be sent to the front, and he was in Camp Cameron before Washington for six weeks. When his regiment returned to New York he received an appointment on the staff of General Frederick W. Lander. He was severely wounded in a skirmish on February 26, 1862, and lingered until April, when he died at Cumberland, Maryland.
From Wikipedia.