Infectious mononucleosis (IM) is an acute infectious illness characterized by swollen lymph nodes, fever, pharyngitis, fatigue, and head and body aches. This report describes the incidence rates, trends, and demographic correlates of IM among active component service members during 2002-2018. During the surveillance period, there were 23,780 incident cases of IM, resulting in an overall incidence rate of 104.2 cases per 100,000 person-years (p-yrs). The incidence of IM diagnoses was highest among the youngest age groups and decreased with increasing age. The rate of incident IM diagnoses was markedly higher among non-Hispanic white service members (123.4 per 100,000 p-yrs) compared to those in other race/ethnicity groups. The incidence of IM diagnoses among recruits (364.9 per 100,000 p-yrs) was 3.4 times that among other enlisted personnel (106.0 per 100,000 p-yrs) and 5.6 times that among officers (64.7 per 100,000 p-yrs). The incidence of IM diagnoses remained relatively stable during the surveillance period, at about 100 per 100,000 p-yrs. IM is not considered to be a serious illness; however, it can seriously impact availability for duty during the acute phase.