Hugh Alexander Pollock was born in Ayr, Scotland, the elder of the two sons of bookseller and publisher William Smillie Pollock (1858–1942) and his wife Jessie Smith McBride. He was educated at Ayr Academy. He and his younger brother Fred worked in his father's business.
He joined the British Army, and became a second lieutenant in the 5th Battalion of the Royal Scots Fusiliers in May 1912. He married Marion Atkinson in October 1913, at the Hotel Dalblair in Ayr. They had two sons, William Cecil Alexander (1914–16) and Edward Alistair (1915–69).
In the First World War, he served with his regiment in Gallipoli, Palestine and France. He was a captain, serving as an adjutant, in September 1915. From December 1915 to May 1916, he had served as commander of the 6th Battalion of the Royal Scots Fusiliers on the Western Front. He had been promoted to major, attached to the 12th Battalion, when he was awarded the Distinguished Service Order in 1919. After the war, he served as a temporary captain in the Indian Army.
After leaving the army Pollock moved to England and joined the publishers George Newnes in London. He worked with Winston Churchill in the 1920s, editing Churchill's six-volume narrative history The World Crisis, published between 1923 and 1931.
Through Newnes Pollock met Enid Blyton, a writer nine years his junior, after she had been commissioned to write a children's book about London Zoo. Their relationship developed, and shortly after he divorced his estranged first wife he married Blyton at Bromley Register Office in August 1924; the couple spent their honeymoon in Jersey. After their marriage the Pollocks lived in a flat in Chelsea. They moved out of central London in 1926 to live at Elfin Cottage in Beckenham, and then to Old Thatch in Bourne End, Buckinghamshire in 1929. They had two daughters: Gillian Mary (1931–2007), and Imogen Mary (b. 1935). The family moved in 1938, settling in a large house in Beaconsfield in Buckinghamshire which was named Green Hedges by readers of Blyton's magazine Sunny Stories.