Background Common mental disorders are the major public healthproblem that affects mothers with young children. Although there were a number of studies done on maternal mental health problems, they were largely focused on perinatal period. However, there is scarcity of information on the magnitude and correlates of these mental health problems beyond perinatal period and due concern is not given mainly in LMICs including our country. Objective To assess the prevalence and factors associated with common mental disorderamong mothers of under-five year children at Arbaminch town, South Ethiopia, 2019. Methods A community based cross-sectional study was conducted in May and June 2019 at Arbaminch town. A systematic random sampling technique was used to select 776 participants. The Self-Reporting Questionnaire (SRQ-20) was used to assess common mental disorder (CMD). Data was coded and entered in EPIDATA3.1 and analyzed using SPSS version 25. Bivariable and multivariable logistic regression were used to identify factors associated to common mental disorder. P-values less than 0.05 were considered statistically significant and strength of the association was presented by adjusted odds ratio with 95% confidence interval. Result The prevalence of common mental disorder among mothers with children aged below five years was 36.6% with (95% CI, 33.2, 39.9). Being single/divorced/widowed [AOR = 3.64, 95% CI:(1.47, 8.99), chronic medical illness [AOR = 3.25, 95% CI:(2.10, 5.04)], exposure to two/more stressful events [AOR = 1.62, 95% CI:(1.11, 2.36)], poor social support [AOR = 2.59, 95% CI:(1.62, 4.14)], mothers living with cigarette smoker husband [AOR = 2.03, 95% CI:(1.19, 3.47)], and mothers physically abused by their spouse [AOR = 2.36, 95% CI:(1.49, 3.74)] were factors associated with common mental disorder. Conclusion and recommendation The prevalence of common mental disorder was high among mothers with children aged below five years compared to the general population. Being single/divorced/widowed, chronic medicalillness, exposure to two/more stressful events, poor social support, mothers living with cigarette smoker husbandand physically abuse by their spouse were factors associated with common mental disorder. Early detection and management of these maternal mental health problems is vital for mothers’ wellbeing as well as growth and development of children.