Born in Beverly, New Jersey, he moved to Philadelphia, Pennsylvania in 1894. Originally trained in law, he turned to journalism, first as a correspondent, and later as editor. He was assistant editor of The American Weekly from 1912 to 1937 under Morrill Goddard, then its editor from 1937 until his death. As editor, he hired the unheralded new artists Virgil Finlay, Hannes Bok, and promoted the work done on polio by Sister Elizabeth Kenny.

His fiction was only a sideline to his journalism career, which might explain his relatively low output. One of the best-paid journalists of his era, Merritt made $25,000 per year by 1919, and at the end of his life was earning $100,000 yearly—exceptional sums for the period. His financial success allowed him to pursue world travel—he invested in real estate in Jamaica and Ecuador—and exotic hobbies, like cultivating orchids and plants linked to witchcraft, magic (monkshood, wolfbane, blue datura—and peyote, marihuana).

Merritt married twice, once in the 1910s to Eleanore Ratcliffe, with whom he raised an adopted daughter, and again in the 1930s to Eleanor H. Johnson. He maintained an estate in Hollis Park Gardens on Long Island, where he accumulated collections of weapons, carvings, and primitive masks from his travels, as well as a library of occult literature that reportedly exceeded 5000 volumes. He died suddenly of a heart attack, at his winter home in Indian Rocks Beach, Florida, in 1943.

Merritt's writings were heavily influenced by H. Rider Haggard and Gertrude Barrows Bennett (writing as Francis Stevens), with Merritt having "emulated Bennett's earlier style and themes." Merritt's stories typically revolve around conventional pulp magazine themes: lost civilizations, hideous monsters, etc. His heroes are gallant Irishmen or Scandinavians, his villains treacherous Germans or Russians (in accordance with the politics of the time) and his heroines often virginal, mysterious and scantily clad.

What sets Merritt apart from the typical pulp author, however, is his lush, florid prose style and his exhaustive, at times exhausting, penchant for adjective-laden detail. Merritt's fondness for micro-description nicely complements the pointillistic style of Bok's illustrations, and often serves to highlight and radicalize the inherent fetish tendencies of pulp sf.In 1917, he published his first fantasy, "Through the Dragon Glass", in All-Story Weekly . This was followed by many more tales, including People of the Pit (1918), The Moon Pool (1919), The Metal Monster (1920), The Face in the Abyss (1923), The Ship of Ishtar (1924), The Woman of the Wood (1926), Seven Footprints to Satan (1927), Burn Witch Burn! (1932), Dwellers in the Mirage (1932), Creep, Shadow! (1934), and The Drone Man (1934).

He also contributed to the round robin story The Challenge from Beyond with Lovecraft, Robert E. Howard, C. L. Moore, and Frank Belknap Long.

The Fox Woman and the Blue Pagoda (1946) combined an unfinished story with a conclusion written by Merritt's friend Bok. The Fox Woman and Other Stories (1949) collected the same fragment, minus Bok's conclusion, with Merritt's short stories. The book The Black Wheel was published in 1948, after Merritt's death; it was written by Bok using previously unpublished material as well.

Merritt's literary reputation has not stood up well over the years among speculative fiction fans and critics

(with the singular exception of ''The Ship of Ishtar'', a universally hailed classic of the fantasy genre),  but at one time he was a major influence on [[H. P. Lovecraft]]"I was extremely glad to meet Merritt in person, for I have admired his work for 15 years. ... he has a peculiar power of working up an atmosphere and investing a region with an aura of unholy dread" H.P. Lovecraft's letter to [[R. H. Barlow]] (January 13, 1934) [http://skullsinthestars.com/2009/02/22/a-merritts-dwellers-in-the-mirage/]