Theimer has conducted the SJU Men's Chorus since joining the faculty at Saint John's University in 1969. In the same year, he also organized the first mixed choir for students of the College of Saint Benedict (a college for women) and Saint John's University (a college for men), which evolved into the CSB/SJU Chamber Choir, now one of the major touring ensembles of the combined CSB/SJU music department.
