William Stanley Beaumont Braithwaite was born in Boston in 1878, the second of five children. His father, William Smith Braithwaite, was a native of British Guiana and his mother, Emma DeWolfe, was the daughter of a mulatto former slave woman. When Braithwaite was six years old, his father died, leaving the family destitute. His father's untimely death caused him to leave school at the age of twelve to seek employment to help support his family. His two sisters, Eva and Rosie, formed a musical duo, “Sadie and Rosie” in the 1890s, toured the United States and Europe and joined Sam T. Jack's famous Creole Show at the turn of the century. In 1902, Braithwaite met Emma Kelly while working in New Hampshire; they married the next year. Three daughters and four sons were born from their union: Fiona Lydia Rossetti (Mrs Merrill Carter), Katherine Keats (Mrs William Arnold), William Stanley Beaumont, Jr., Edith (Mrs Carman Agard), Paul Ledoux, Arnold DeWolfe and Francis Robinson.

Braithwaite worked for several years as an errand boy and porter, eventually finding employment in the press rooms of Ginn and Company Publishers, where he worked his way up to compositor. It was here that his interest in books developed and he began to write poetry. Braithwaite published his first volume of poetry Lyrics of Life and Love in 1904, after guaranteeing the cost of printing. He sold the small volume door to door. Four years later his second book of verse, The House of Falling Leaves, was published. He also wrote critical essays and occasional poems which were published in periodicals, among them the Atlantic Monthly, the and Scribner's. In 1906 he began his twenty-five year career as a literary critic for the influential Boston Evening Transcript. During this period he edited several books of poetry, including The Book of Elizabethan Verse (1906), The Book of Georgian Verse (1908) and The Book of Restoration Verse (1909). Braithwaite is best known, however, for the annual anthologies of magazine verse he edited from 1913 to 1939. The early works of Carl Sandburg, Edgar Lee Masters and Vachel Lindsay, among others, appeared in these volumes. In addition he edited: Golden Treasury of Magazine Verse (1918); The Book of Modern British Verse (1919); The Story of the Great War (1919), a children's book; Victory! Contributed by 38 American Poets, with an introduction by former President Theodore Roosevelt; Our Lady's Choir, a Contemporary Anthology of Verse by Catholic Sisters (1931), and The Book of Massachusetts Poets. He also published a volume of his critical writings, The Poetic Year for 1916 (1917). In the early 1920s he founded and headed a publishing firm, B.J. Brimmer Company, which published works by Lucius Beebe, Georgia Douglas Johnson and the early novels of James Gould Cozzens.

In 1918 Braithwaite was awarded the National Association for the Advancement of Colored People's Arthur B. Spingarn Award for outstanding achievement in the field of literature. Talladega College and Atlanta University conferred honorary degrees on him, also in 1918. He accepted a professorship in creative literature at Atlanta University in 1935, and remained in this post until his retirement in 1945. In 1948 Cowann McCall published his Selected Poems. Two years later, his biography The Bewitched Parsonage: the Story of the Brontes was published to critical acclaim. The book sold poorly, however, and he was unable to find a publisher and the necessary advances to complete several tentative works, including a biography, and a critical anthology of teaching, for which detailed outlines and initial chapters were already written. His last published work, Anthology of Magazine Verse for 1958, co-edited with his friend and writer Margaret Carpenter, was a retrospective anthology from his previous compilations combined with a selection of recent poems.

In his poetry, Braithwaite played on the traditional romantic themes of nineteenth-century British verse and employed traditional forms and meters. During the Harlem Renaissance, he contributed several critical essays on Black poetry, including Alain Locke's anthology, The New Negro (1925). But the body of his critical work and compilations lacked a clear racial identity. This explains, in part, why he has fallen into relative obscurity in the era of African-American studies.

William Stanley Braithwaite died at his home in Harlem on June 8, 1962.