After serving on the faculty at Calvin College, Armstrong returned to St. Olaf in 1990 and as conductor of the St. Olaf Choir has toured throughout the United States and to Denmark, Norway, Australia, New Zealand and Central Europe. The choir has recorded 12 CDs during Armstrong's tenure as conductor. Armstrong holds membership in a number of professional societies and is active as a guest conductor and lecturer throughout North America, Europe, Scandinavia, Korea, Singapore, Australia, New Zealand and the Caribbean.
Armstrong is widely recognized for his work with youth and children's choral music. He began his tenure as conductor of the Oregon Bach Festival Youth Choral Academy in June 1998. In the summer of 2001, he served as co-conductor of the World Youth Choir sponsored by the International Federation of Choral Music. He served for more than 20 years on the summer faculty of the American Boychoir School in Princeton, N.J., and was conductor of the St. Cecilia Youth Chorale, a 75-voice treble chorus based in Grand Rapids, Mich., from 1981 to 1990.