Objective In 2015, the Australian Government introduced several mental health reforms, including the requirement that Primary Health Networks (PHNs) provide stepped care services for Australians with mental health needs such as anxiety and depression. This paper reports on the development and feasibility study of StepCare, an online stepped mental healthcare service in general practice that screens patients, provides immediate feedback to patients and general practitioners (GPs), transmits stepped treatment recommendations to GPs and monitors patients' progress, including notification of deterioration. Methods The present codesign and feasibility study in one PHN examined: (1) the acceptability and feasibility of StepCare to GPs, practice staff and patients; (2) the impact of StepCare on clinical practice; and (3) the barriers to and facilitators of implementation. Results Thirty-two GPs, 22 practice staff and 418 patients participated in the study. Overall, patients, practice staff and GPs found StepCare acceptable and feasible, commending its privacy, the mental health screening, monitoring and feedback. They also made suggestions for service improvements. GPs reported that StepCare helped with their identification and management of patients with common mental health issues. Conclusions Preliminary data suggest that StepCare may be acceptable and feasible in Australian general practice, helping GPs identify and manage common mental health problems in their patients. The study provides implications for policy and practice, and points the way to future translational research into stepped mental health care. What is known about the topic? Depression and anxiety are common illnesses in primary care and GPs are ideally placed to implement stepped care approaches enabling early detection and accessible, effective care. What does this paper add? Developed in and for general practice, StepCare is the first fully integrated stepped approach to primary mental health care in Australia. As a first step in a translational research program evaluating the effectiveness of StepCare, this paper reports data regarding the feasibility and acceptability of the service. What are the implications for practitioners? Integrated into the workflow of general practice, StepCare is an online service that helps GPs detect new cases of depression and anxiety, provide evidence-based stepped care treatments and monitor patients' progress.