Arnold was born in Swanscombe, Kent, as son of Sir Edwin Arnold. Most of his childhood was spent in India, but he returned to England to study agriculture and ornithology. He became a journalist in 1883, and published his first books A Holiday In Scandinavia (1877) and Bird Life In England (1887) before writing his first novel The Wonderful Adventures of Phra the Phoenician, the adventures of a warrior who goes in and out of an unexplained state of suspended animation in order to be a witness to invasions or attempted invasions of England. Phra was first published in 24 parts in the prestigious Illustrated London News, and later published in book form in the United States and the United Kingdom.
Arnold later wrote other novels, including Rutherford the Twice-Born (1892) and Lepidus the Centurion: A Roman of Today (1901), both of which flopped commercially. In 1905 Arnold published his best known novel, Lieut. Gullivar Jones: His Vacation, also known as Gulliver of Mars, (1905). Its initial reception was lukewarm, leading Arnold to stop writing fiction altogether.