During his five years at the Royal Academy of Music, Bax was deeply impressed by the poetry of W.B. Yeats, founder of the Irish National Theater, an influence that led to a close association with Celtic culture and legend for the rest of his life. He wrote poetry under the pseudonym Dermot O'Byrne, and assisted his brother, the playwright and critic Clifford Bax, in editing a magazine called Orpheus, dedicated to the mystical arts.
His first mature work, In the Fairy Hills, is typical of the fantastic and exotic nature of his orchestral writing, chromatic and opulent, with a broad melodic sweep and luminous harmonies. The Garden of Fand (1916), an imaginative evocation of an ancient legend of sea gods and goddesses, is similarly impressionistic, though less naturalistic, than Debussy's La Mer. Tintagel, a tone poem inspired by traditional English stories of King Arthur and his Knights of the Round Table was composed in 1919 after a holiday in Cornwall and quickly became Bax's most frequently performed work. Living in the shadow of composers of the stature of Elgar and Vaughan Williams, Bax received little public recognition until late in life. Up to the late 1930s, his songs, choral works, and chamber music were rarely heard, and, had it not been for a broadening of his style and the championship of Sir Adrian Boult, conductor of the BBC Symphony Orchestra, Bax would probably be remembered, if at all, for his comparatively youthful works. Even in the 1960s the English music critic Burnett James was moved to call this neglect "myopic and moronic." On a visit to Scandinavia in 1932, Bax met Sibelius and the two composers became friends; while Sibelius' influence is not obvious in Bax's symphonic style, he is clearly indebted to the Finnish master in Winter Legends and The tale the pine trees knew. The symphony, a form to which he turned again and again between 1922 and 1939, provided an outlet for a more taut, structured and contrapuntal approach that nevertheless retains elements of fantasy and mysticism. Symphony No. 1, the only one recorded in his lifetime, was first performed in 1922, and followed by six more. The fifth is dedicated to Sibelius and the sixth contains a theme from Sibelius' tone poem Tapiola.
Bax did not take well to approaching old age. He became withdrawn and dependent on alcohol. In 1943, he wrote a bitterly nostalgic memoir of his earlier years called Farewell to my youth (edited by Lewis Foreman; Scolar [sic] Press (Now Ashgate); 1992). In 1942, he was appointed Master of the Kings' Music and received a knighthood. His last work, written to celebrate Queen Elizabeth II's coronation in 1953, is a set of madrigals called What is it like to be young and fair. He died while on holiday in Cork, Ireland.