The choir at the west end of St Paul's was extended in 1909, designed by Hugh Vallance.

[6][7] The church building was designed by William Thomas (architect) in the style of English Gothic revival.

The building's exterior is grey limestone which comes for the most part from local Hamilton quarries by stonemason George Worthington.

Architectural historian Marion MacRae assessed the building as "the best Decorated Gothic Revival Church in Ontario".

It rises to a height of 100 feet to the top of the parapet line at which point it almost imperceptibly reduces itself into an octagonal spire with lucarnes on alternate sides.