Surprisingly for a writer so closely associated with the sea, and for a man who successfully lobbied on behalf of the conditions of working seamen, Russell spent only eight years in the merchant navy. He was born in the Carlton House Hotel in New York City on 24 February 1844, the third son and fourth child of the composer and musical entertainer Henry Russell. Though sometimes identified in sources as an American novelist he had no American blood in his veins and his birth there was entirely owing to his father's musical career. Henry Russell is most famous as the composer of ‘A Life on the Ocean Wave’ and other sea shanties but he earned his living as a singer and performer whose concerts made him a kind of ‘musical counterpart of Charles Dickens’. At the time of his new son's birth he was coming towards the end of a year-long tour across the United States and the young William Clark was less than three months old when he made his maiden voyage, sailing for England with his parents on 17 July 1844.

Henry Russell came from a Jewish family but William Clark's mother, Isabella Lloyd, was a Quaker descended from the wealthy founders of Lloyd's Bank. The children were not brought up in the Jewish faith and spent little time with their father.