Brother of Edith and Sacheverell, born and brought up at Reinshaw Hall, Derbyshire (which later he inherited) and son of Sir George Sitwell. He went to Eton and served in the Grenadier Guards.

Until the mid-1920s, when Sitwell met David Horner, who would become his long-time partner, there is no mention anywhere of Sitwell's sexuality. Sitwell's discretion did not seem to stem from shame or fear of the opinion of others: he was not furtive or deceptive about his homosexuality, and made no pretences about his relationship with the flamboyant Horner.

He published his Selected Poems (1943), and wrote art criticism, novels including A Place of One's Own (1941), and a series of autobiographical volumes. He reluctantly served in WWI, and his early poetry is sharply satirical and pacifist in tone. He also wrote books describing his travels in Italy, China and the Far East. Sir Osbert Sitwell, who never married, died after succumbing to Parkinson's Disease.