Born in the crippling poverty of early 18th Century Spitalfields, his family evidently feared that he would follow his older brother into an early grave, having him baptised the day after his birth. Despite their fears the boy lived, but tragedy still struck the young family as both his younger sister and father died while he was young.
Unable to support her children unaided, his mother sent Jack to the workhouse at Bishopsgate and from there he was apprenticed as a carpenter (as his father had been). By the time he was 20 he was showing great promise in this career and seemed to have the prospect of a fine career within sight but, like many young men before and since, was also fond of the ale-house. During his visits to the pubs and inns of Drury Lane, he began to associate with less savoury elements who also frequented the establishments and his dedication to his career began to wane.
Among his friends were Joseph "Blueskin" Blake and Jonathan Wild. Blake would become Sheppard's criminal accomplice, whereas Wild would become his nemesis in his role as the self-proclaimed "Thieftaker".
Exactly how Sheppard drifted in crime is uncertain. According to his own autobiography (possibly ghostwritten by fellow Newgate inmate Daniel Defoe) he fell in with both drink and a prostitute named Elizabeth Lyon and from there he began a downward spiral into criminality. Some modern biographers contend that actually he was merely tired of life as a labourer and attracted to the creed of the Levellers.
Either way, by his mid twenties he was throwing away his career in carpentry in order to supplement his income. In 1723, he was detected shoplifting - stealing silver spoons from his master's client in Charing Cross. This swiftly became a pattern as he stole from clients while working. Progressing to burglary, he became increasingly involved with the gang run by Wild and he quit his career in August 1723 to move to Picadilly with Lyon to live as man and wife.
When she was arrested for prostitution and incarcerated at St. Giles's Roundhouse. When he was refused visiting rights, Sheppard broke into the gaol and liberated her, knocking down the beadle and smashing down the door to her cell.