Aboriginal and Torres Strait Islander women experience higher rates of obesity, chronic disease, and are less active than non-Indigenous Australian women. Lifestyle programs designed to increase physical activity and encourage healthy eating are needed to ameliorate this disparity. The aim of this study was to identify participants' perceived barriers and enablers to attend group exercise classes as part of a 12-week fitness program. To understand the factors that influence attendance, a mixed method process evaluation was undertaken in which a quantitative measure of attendance in the group exercise classes was used to identify cases for further qualitative investigation. Aboriginal and/or Torres Strait Islander women aged 18 to 64 years were recruited to a research trial of a fitness program. The 12-week program included two 60-minute group exercise classes per week, and four nutrition education workshops. Semi-structured interviews were conducted at program completion. Participants were stratified by attendance, and interviews from the highest and lowest 25 percentiles analysed. Rigour was strengthened through use of multiple data analysts, member checking and prolonged engagement in the field. Analyses of the post-program interviews revealed that participants enrolled in the program primarily for the perceived health benefits and all (with one exception) found the program met their needs and expectations. The atmosphere of classes was positive and comfortable and they reported developing good relationships with their fellow participants and program staff. Low attendees described more barriers to attendance, such as illness and competing work and family obligations, and were more likely to report logistical issues, such as inconvenient venue or class times. Attendance to the 'Aboriginal and Torres Strait Islander Women's Fitness Program' was primarily influenced by the participant's personal health, logistics and competing obligations. Low attendees reported more barriers during the 12-week period and identified fewer enabling factors than high attendees. Australian New Zealand Clinical Trials Registry ACTRN12610000224022.