Mr. Clas opened his own office, and practiced alone until 1890 when he joined the late George B. Ferry in organizing the firm of Ferry & Clas, which during the next twenty years became one of the leading architectural offices in Milwaukee. In that period the partners designed a number of important buildings in the city, including: the Public Library and Museum; the Plankinton Hotel; the City Auditorium; Tripoli Theatre; the City Hospital; Northwestern National Insurance Building; and tower of St. John's Cathedral. Ferry & Clas were also architects of the State Historical Society's Library at Madison, and the Public Library at Jackson, Mich.
In 1913 the firm of Clas, Shepherd & Clas was organized, with Reuben F. Clas and John S. Shepherd as junior partners. Work was executed under that name until 1931 when Mr. Shepherd withdrew, and the firm was reorganized as Clas & Clas, Inc., with the elder architect remaining president until the time of his death. A leading member of the profession, known throughout the state, Mr. Clas held many honorary positions, also was active City Planning work in Milwaukee, having been one of the sponsors of the Civic Center Plan, 1911, and a member of the County Planning Board, in addition he served as Advisor to the County during the erection of the Court House,, and to the State at the Capitol and on the State Reformatory at Creen Bay.
He was a charter member of the Wisconsin Chapter, A. I. A. in 1911, and continued that association during his long career and until the time of his decease at the age of eighty-two.