To date, there have been no cohort studies of sexual harassment incidence and its relation to mental health within humanitarian field-workers. Research among numerous occupations suggests an association between workplace sexual harassment and several health complaints. This study examined the incidence and severity of sexual harassment and its association with changes in mental health in a cohort of international humanitarian aid field-workers (iHAWs). Four hundred and seventy-eight iHAWs filled in questionnaires about sexual harassment, depression, anxiety, and Post-Traumatic Stress Disorder (PTSD) as part of a larger study on health and well-being. Six percent of male and 18% of female iHAWs reported experiencing sexual harassment during their latest field assignment, with most reporting low levels of nonphysical forms of sexual harassment. Sexual harassment was predictive of negative changes in both depression and anxiety symptom severity between before and after an assignment for females; for males, it predicted negative changes in PTSD symptom severity. Sexual harassment did not predict utilization of mental healthcare services within 2 months after the end of assignment. The current findings are the first figures derived from a representative sample of iHAWs on the incidence of sexual harassment during a field assignment and show sexual harassment to be a relatively common and present issue. The findings are mostly in line with the extant literature and underscore the importance of attending to the issue of sexual harassment in the humanitarian sector.