John Plaw was baptized 8 January 1746 in Putney, London, England. He studied to become an architect under Thomas Kaygill in London and became an established builder and architect in Westminster. However, many of his commissions were for country buildings and these became his specialty. In 1806 or 1807, John immigrated to Prince Edward Island with his wife Mary, her sister Betsy Ball, and his nephew Joseph Ball, who would later became a prominent Island surveyor.

As Architect to the Island government, Plaw submitted plans for a new jail in 1809, several designs for the proposed court house in 1810 as well as plans and designs for roads. He also acted as surveyor during Thomas Wright's absences and gave lessons to carpenters on the principles of geometry and architectural drawing. Plaw designed a circular store for Waters and Birnie, a London-based firm doing business in P.E.I., but it is unclear whether these plans were ever implemented. He also designed a 16-sided market house on Queen's Square which replaced the 1813 Market House. The "Round Market", as it was commonly known, was built by Isaac Smith, Henry Smith, and Thomas Hodgson in 1823, three years after Plaw's death. John Plaw died on 24 May 1820 in Charlottetown.