MacKay designed a variety of religious, civil, and private constructions across the Washington Territory and British Columbia.
After entering a commercial dispute with Seattle Catholic parish leader Francis X. Prefontaine, he relocated to Tacoma, Washington, where he saw little business.
The building's transepts and chancel were later enlarged, but MacKay's bell tower, nave, and entrance facade remain largely original.
From 1882 to 1883, he designed the Providence Hospital, possibly with plans laid out by Mother Joseph, and an enlargement of the Roman Catholic Church of Our Lady of Good Help.
[2][4][5] In addition to his work in Seattle, MacKay designed two major projects in Vancouver, Washington, from 1883 to 1885; the First Presbyterian Church and the Saint James Roman Catholic Cathedral.
None of his contracts in Tacoma ultimately came to fruition, beyond a now-demolished Stick style residence for businessman Edward Slade "Skookum" Smith constructed from 1884 to 1886.