Patrick Charles Keely was arguably America’s greatest nineteenth century Catholic architect. He was responsible for the designs of hundreds of Catholic churches and cathedrals across the north-east of the region throughout the century. He was also instrumental in transforming contemporary Catholic church designs, when he introduced Gothic Revival styles to Catholic church building in the 1840s. By categorising and documenting the styles of the notable Catholic churches that were constructed during the period, this research demonstrates that not only did Keely introduce Gothic Revival architecture to Catholic church building, but he also popularised it to the extent that it became the dominant style for Catholic church construction for the rest of the century. He also continuously developed the style in line with changing architectural concepts and fashions and modified his own version of the Gothic style, to make it affordable for less affluent congregations.