A native of Somerset, Deken was admitted to the freedom of London in 1393 as a pewterer, but having shown that he and his former master, Richard Spencer, had always used the Grocers’ rather than the Pewterers’ Mystery, he was re-admitted to the freedom as a grocer four years later, upon payment of the standard 40s. fine. By 1401 he was rich enough to afford his own livery and became a member of the company’s ‘inner fraternity’. His growing commercial interests may well have taken him abroad, for in May 1403 a John Deken and four others were about to leave the country in order to begin litigation then considered prejudicial to the King and many of his subjects. In September 1407 the same man did indeed appear among the plaintiffs who successfully brought a suit before the court of the marshal of Calais regarding the unjust detention of a debt, although a royal commission was subsequently empowered to hear an appeal against the judgement.
Deken’s first election in May 1415 as a warden of the Grocers’ Company — then one of the wealthiest and most powerful of the London livery companies — is a clear indication of his rising fortunes. So too is the frequency with which, from 1420 onwards, he was called upon to witness local conveyances of property, sometimes concerning the enfeoffment of lands or rents for the upkeep of London Bridge, and often in association with William Redstone, his colleague in the Parliament of May 1421.
Having established himself as a man of considerable influence in Southwark and the City, Deken was also involved in the affairs of other prominent merchants, including William Brampton I and William Sevenoak, two of the most powerful men in London. In July 1426 he joined with Thomas Wykes of Kent in offering sureties of £200 to John, duke of Bedford, for the payment of £177 in damages owed by the abbot of St. Augustine’s, Canterbury, to certain foreign merchants, but he was discharged from his obligations in the following January.