Walter John de la Mare was born to James Edward de la Mare, a clerk at the bank of England, and Lucy Sophia Browning (James' second wife), daughter of Scottish naval surgeon & author Dr Colin Arrott Browning. The assertion that Lucy was related to poet Robert Browning has been found to be incorrect. He had two brothers, Francis Arthur Edward and James Herbert ('Bert'), and four sisters Florence Mary, Constance Eliza, Ethel (who died in infancy), and Ada Mary ('Poppy'). De la Mare was known as Jack by his family and friends as he hated the name Walter.

In 1892, De la Mare joined the Esperanza Amateur Dramatics Club, where he met and fell in love with Elfrida (Elfie) Ingpen, the leading lady. Elfie was ten years older than De la Mare. On August 4, 1899 De la Mare and Elfie, who was by then pregnant, were married. They went on to have 4 children – Richard Herbert Ingpen ('Dick'), Colin, Florence and Lucy Elfrida ('Jinnie') de la Mare. In 1940 Elfie was diagnosed with Parkinson's disease and spent the rest of her life as an invalid, eventually dying in 1943. From 1940 until his death, De la Mare lived in Montpelier Row, Twickenham, the same street where Alfred Tennyson, 1st Baron Tennyson had lived a century earlier. In 1947 he suffered from coronary thrombosis and died of another in 1956.

De la Mare's play Crossings has an important role in Robertson Davies' novel The Manticore. In 1944, when the protagonist David Staunton is sixteen, de la Mare's play is produced by the pupils of his sister's school in Toronto, Canada. Staunton falls deeply in love with the girl playing the main role – a first love which would have a profound effect on the rest of his life.