With walls of Brandywine granite and a soaring, sixty-five-foot buttressed tower and spire, St. John's stands as a symbol of stability in a deteriorated neighborhood. It is the final documented church by Notman, noted Philadelphia architect. Alexis I. du Pont, congregant at Trinity (WL47, but then at 5th and King sts.) and recent convert to high-church Anglicanism, led the way in establishing this offshoot in the flourishing residential district north of the river. It stood at the corner of two important roads and replaced the notorious Green Tree Inn, the bar of which supposedly lay where the altar is now. Notman employed features advocated by the Ecclesiological movement: orientation to the east, cruciform plan, deep chancel, and exposed wooden beams. The style is Early English Gothic with Decorated touches and recalls his St. Mark's, Philadelphia (1847–1852).
“We are today building better churches than any other people in the world, with the exception of England,” said Gothic Revival architect Ralph Adams Cram in 1929, pointing to such Philadelphia firms as Zantzinger, Borie and Medary, creators of St. Paul's, Chestnut Hill, Philadelphia (1928–1931), and the exquisite Valley Forge Memorial Chapel in Pennsylvania (1917), as well as Rodney Square (WL29). They skillfully added to St. John's in 1919–1921. In the chancel, the arches of the sedilia (seats) have carved ornament even on their not-visible inner sides—meant for the glorification of God, not gratification of man. Additions at that date, in English Perpendicular style, included the Lady Chapel, Delaware's finest example of Cram-derived Gothic Revival. Wood carving was provided by American Car and Foundry Company. The splendid organ screen was carved in oak by Belgian-born sculptor Edward Maene of Philadelphia, who created the Valley Forge work, too. The Zantzinger firm's work here bears further comparison to their Foulke and Henry Dormitories, Princeton, and Philadelphia Divinity School (all 1923). The church has served as the diocesan cathedral since 1935, the year one of several new stained glass windows by Philadelphian Henry Lee Willet was dedicated.