The material in this collection represents Mueller's work from his first years as a pastor in the Evangelical Association to his retirement from the Episcopacy of the United Methodist Church (1921-1975). The earliest manuscripts include sermons and addresses (1921-1975). The rest of the collection consists of correspondence (1941-1973), photographs (1962-1976), addresses (1921-1975), articles, and clippings (1921-1975). The most sustained record of Mueller's work begins in the 1940's with Mueller's election as Executive Secretary of Christian Education. The collection continues with records from his episcopal leadership, his presidency of the N.C.C., and his involvement in the negotiations for the Uniting Conference in 1968.