He was educated at Rutgers and Columbia universities. His first volume of verse, Summer of Love (1911), showed the influence of William Butler Yeats and the Irish poets. After his conversion to Catholicism, Kilmer attempted to model his poetry upon that of Coventry Patmore and the 17th-century Metaphysical poets. His most famous poem, “Trees,” appeared in Poetry magazine in 1913. Its immediate and continued popularity has been attributed to its combination of sentiment and simple philosophy. His books include Trees and Other Poems (1914); The Circus and Other Essays (1916); Main Street and Other Poems (1917); and Literature in the Making (1917), a series of interviews with writers. Kilmer joined the staff of The New York Times in 1913. In 1917 he edited Dreams and Images, an anthology of modern Catholic poetry. Kilmer was killed in action during World War I and was posthumously awarded the Croix de Guerre.