From my new book, Almanac of Architecture and Design, it says the KCMO area (outside metro also) has the 2 oldest still active architectural firms in Missouri.
Oldest Architectural Firms in MO:
1. St. Joseph Brunner & Brunner Architects & Engineers 1872
2. Kansas City, MO The Austin Company 1878
3. Kansas City, MO Burns & McDonnell 1898
4. St. Louis, MO William B Ittner, Inc. 1899