Swinnerton was born in Wood Green, a suburb of London, the son of Charles Swinnerton, a copperplate engraver, and Rose, née Cottam.
Swinnerton left school at the age of 14 and was employed as an office boy for a newspaper publisher, Hay, Nisbet & Co and then as a clerk-receptionist by J. M. Dent, publishers of Everyman's Library. He moved on to the publishing house of Chatto & Windus, first as a proof-reader and then as an editor. Although he began writing novels in 1909, he continued editing until he became a full-time author in 1926. Even then, he also worked as literary critic for the magazine Truth, the London Evening News (1929–32) and The Observer (1937-43).
As a novelist, Swinnerton achieved critical and commercial success with Nocturne in 1917, and remained a successful writer for the rest of his life. His last novel, Some Achieve Greatness (1976), was published when he was in his early nineties. Some critics detected echoes of George Gissing and Arnold Bennett in Swinnerton's work, but he himself thought his chief influences were Henry James, Henrik Ibsen and Louisa May Alcott.
His prose style was "natural and lucid", and he was disapproving of over-intellectual or pretentious writing. In The Georgian Literary Scene, an evocation of the era of the gentlemanly man of letters in its final years, he wrote, "If I dwell for a moment longer, as I fear I must, upon the weakness of too much scholarship in the arts, it is because I think scholarship is nowadays excessively valued as a necessary preliminary to creative writing." Of all of his critical contributions, The Georgian Literary Scene stands out, and it is still used by those who study the period. The New York Times declared it "wholly – and most refreshingly – unlike other literary histories." Swinnerton himself said of his work: "My best books, in my own opinion, are Harvest Comedy and The Georgian Literary Scene, but I do not regard either one as of lasting importance.... I live in the country, am very lazy, work unwillingly very hard, and have few intolerances."
Swinnerton lived for more than fifty years in Old Tokefield, Cranleigh, Surrey, a rural spot not far from London. He was twice married; his first marriage, in 1917, to the poet Helen Dircks, ended in divorce. In 1924 he married Mary Dorothy Bennett, with whom he had one daughter. Swinnerton died at Old Tokefield at the age of 98. His obituary notice in The Times began by noting that his death "breaks one of the last links with his great contemporaries, Wells, Galsworthy and Arnold Bennett."