Online parenting interventions hold promise for increasing access to behavioral support for families with low income and who reside in rural areas. The current study evaluates the efficacy of a mobile app-based parenting support program, the Family Check-Up Online (FCU-O) with telehealth coaching support, for parents of children 1.5-5 years old at risk of experiencing parenting challenges. We tested effects of the FCU-O on parent wellbeing (parenting self-efficacy and symptoms of depression and anxiety) and parent-reported parenting skills (support for positive behavior, limit-setting, and proactive parenting) at the 3-month follow-up in a randomized controlled trial (FCU-O versus control). We also tested relationships between parents' baseline characteristics and their engagement with the app and telehealth coaching. Eligibility criteria included endorsing depressive symptoms or current or past substance misuse. Participants (N = 356) were predominantly White (72%) and low-income, with 43% of families residing in a rural area. Approximately one-third-of participants reported clinically significant symptoms of anxiety or depression and one-third-endorsed a lifetime history of opioid misuse. Intent to treat analyses tested effects of the FCU-O on parent wellbeing and parenting skills. The FCU-O was associated with improvements to parents' support for positive behavior, limit-setting, and proactive parenting skills, as well as parenting self-efficacy and depressive symptoms. Parents' initial symptoms of depression and anxiety significantly predicted greater telehealth coach engagement; low levels of initial self-reported skills positive behavior support and limit-setting significantly predicted greater app engagement. Results provide support for the FCU-O as a preventive parenting intervention for parents experiencing mental health challenges, with implications for reducing barriers to accessing parenting support for underserved families.