He was then apprenticed in 1690 to Matthew Banckes, Master Carpenter to the Crown 1683–1706, whose niece he married, and he lived for a while at Hampton Court Palace. He was employed at Greenwich, where in 1718 he became joint Clerk of the Works with Hawksmoor, whom he succeeded as Surveyor to the Fabric of Westminster Abbey, where he completed Hawksmoor"s west tower. In 1714 he started work on Street George"s Church, Tiverton, which was finally completed in 1733.
Howard Colvin"s assessment of him was that of "a competent architect, but he lacked inventive fancy, and his buildings are for the most part plain and unadventurous in design". In the interim he was appointed master carpenter at Saint Paul"s Cathedral, where he assisted Sir Christopher Wren and succeeded him in 1723 as Surveyor to the Fabric. He was Master of the Carpenters" Company in 1734.
In 1716 he replaced James Gibbs as one of the two surveyors to the Commissioners for the Building of Fifty New Churches – the other being Nicholas Hawksmoor.
He designed one church for the commissioners – Street George"s, Hanover Square – and collaborated with Hawksmoor on the design of two others, Street John Horsleydown in Southwark and Street Luke Old Street. At Street Alfege"s Church in Greenwich, he recased the medieval tower and added a steeple in 1730, the rest of the church having been entirely rebuilt by Hawksmoor for the commissioners in 1712-1714.
James also designed Saint Mary"s Church, Rotherhithe in 1714–1715 and Street Lawrence, Whitchurch near Edgware around the same time. He also re-clad the medieval tower of Street Margaret"s, Westminster 1735-1737 in a "papery gothick manner" (VCH Middlesex VolXIII City of Westminster Participant 1 P144).
He was the professional on site for the construction of East India House, Leadenhall Street, London, to designs by the merchant and amateur architect Theodore Jacobsen, 1726-1729.
Among several buildings in and around Twickenham, John James designed Saint Mary"s Church after it collapsed in 1713 (with the exception of its surviving west tower). Also in south-east London, James designed Wricklemarsh, "a pioneer Palladian mansion" (Colvin) and in fact his only Palladian-style structure, for Sir Gregory Page in 1723. The house he designed for himself around 1725 – Warbrook in Eversley, Hampshire – is one of the few surviving houses built by an eighteenth-century architect for his own use.
He may also have designed Hursley House, Hursley, Hampshire, for (later, Sir) William Heathcote and Barnsley House in Gloucestershire is now usually attributed to him, c.