He was born in East Orange, New Jersey. He attended Kent School in Kent, Connecticut and graduated from Harvard in 1917 after which he went to France and volunteered with the S.S.U. 60 of the Norton-Harjes Ambulance Corps serving the Allied Forces in World War I.
He was a brother of the Epsilon chapter of St. Anthony Hall at Trinity College in Hartford, CT.
His work is in meter and often rhyme. He is known for his sonnets and for such poems as "Theme and Variations" (on his war experiences) and the light "Letter to Robert Frost". The Collected Verse of Robert Hillyer (1934) won the Pulitzer Prize for Poetry in that year. Other works include The Coming Forth by Day: An Anthology of Poems from the Egyptian Book of the Dead (1923), the novel Riverhead (1932), and In Pursuit of Poetry. American composer Ned Rorem's most famous art song is a setting of Hillyer's "Early in the Morning".
Hillyer is remembered as a kind of villain by Ezra Pound scholars who associate him with his 1949 attacks on The Pisan Cantos in the Saturday Review of Literature which sparked the Bollingen Controversy.
Hillyer was identified with the Harvard Aesthetes grouping.