The pair of elaborate, towered, orangish-red brick Italianate and Second Empire houses with mansard roofs stand side by side on the crest of a hilltop overlooking the St. Clair River. The spectacular site originally extended to the river. The houses have ornate bracketed window hoods and cornices. Brothers William S. and Mark Hopkins probably built them with support from their relative Mark Hopkins, who lived briefly in St. Clair before moving to California. In California he made a fortune from a tool business he established in the goldfields of California and in his interest in the Union Pacific Railroad Company. The William Hopkins House on the south, now William Hopkins Manor, seems inspired by Mark Hopkins's house in Menlo Park, California, but in miniature. Architecture professor Emil Lorch and local historians attribute the design of the Mark Hopkins House here in St. Clair to Charles Marsh, a skillful southeastern Michigan architect, who came from Rochester, New York, to Detroit probably by 1875.