Jennings is an alumnus of St. Olaf College and sang as a member of the St. Olaf Choir as an undergraduate. He received his master's degree from Oberlin College and his doctorate from the University of Illinois. Jennings was appointed to the faculty of St. Olaf College in 1953, and ascended to one of the world's most coveted choral podiums in 1968: Jennings was the third director of the internationally renowned St. Olaf Choir, succeeding founder F. Melius Christiansen and his son-successor, Olaf C. Christiansen. He retired from St. Olaf College in 1990, turning over the podium to his former student, Anton Armstrong. His Son, Dr. Mark Jennings, is the director of Choral Activities at Truman State University in Kirksville, Missouri.

As director of the St. Olaf Choir Jennings became noted for guiding and maturing the St. Olaf Choir from one rooted substantially in its 1911 era founding to arguably one of the most highly respected choral ensembles of the world.

While Jennings never forsook the choir's Lutheran heritage, he successfully broadened the ensemble's repertoire to include works of noted composers of the post-World War 11 era.

Since his retirement from St. Olaf in 1990, he has served as a visiting professor and choral conductor at Gustavus Adolphus College in St. Peter, Minnesota and The University of Arizona, Tucson. Jennings served as the guest conductor at anniversary concerts in honor of F. Melius Christiansen.