Swasey arrived in St. Louis in 1885. He and Charles K. Ramsey formed a partnership that same year, then opened his own office in 1887. Almost immediately, examples of his work were published in major architectural magazines. Swasey designed the Pastime Athletic Club opposite Vandeventer Place and a number of impressive Queen Anne and Shingle Style houses before embracing, and perhaps introducing to St. Louis, the "Colonial" style with his work for General Joseph Scott Fullerton, a veteran of the Union Army and a retired attorney. Fullerton´s Westminster Place (the 4300 and 4400 blocks of Westminster) opened in 1892 as a private street with Swasey in charge of the design of the entrance gates and all of the developer-built houses.

In addition to Swasey´s 14 designs for Fullerton in Westminster Place between 1892-95, the young architect began to attract the more baronial commissions in Portland and Westmoreland Places: #13 Portland for William K. Bixby in 1893, #1 Westmoreland for J. C. Van Blarcom in 1894, and #29 Westmoreland for Judge Elmer B. Adams in 1895, among others.

For his own house in Fullerton´s Westminster Place, Swasey first designed an elaborate variation on the Shingle Style at 4382 Westminster. During construction, he decided that it was too large for his small family and purchased the lot next door. A more modest Neo-Colonial house was completed in 1893 at 4384 Westminster and served as the family residence until 1897, when the Swaseys moved to his Westminster Apartments at the northwest corner of McPherson and Newstead.

In 1903, Swasey became the largest investor in the Parkview Realty and Improvement Company, with an agreement from the developers that he would design at least 20 of the houses in the new subdivision. Instead, he moved to New York City to design theatres for the Schuberts.