Lee also had a remarkably early entry into his professional career. He earned a teaching certificate at Albion State Normal School in southern Idaho, and in September 1916 he was appointed, at the tender age of seventeen, to be principal of the Silver Star School at Weston, Oneida County, Idaho. Here, in a one-room school, he had from twenty to twenty-five pupils, ranging from the first to the eighth-grade level. One year later, at age eighteen, he was appointed principal of the larger grade school at Oxford, Idaho, where he served for three winters, including the severe influenza epidemic season of 1918.
The assumption of such major responsibility in his teen years prepared Harold B. Lee, at age twenty-one, for missionary service in the LDS Church. In 1920 he was called to the Western States Mission, headquartered in Denver. In the center of his missionary Bible, Elder Lee recorded the names of the forty-five converts he baptized in two years. His proselytizing time was limited because, again moving ahead rapidly, he became a conference president (presiding over both missionaries and church members in Denver) only nine months into his mission.