People with severe mental illness often have difficulty sustaining employment. Work Behaviour Inventory and Work Environment Impact Scale are vocational assessments used by occupational therapists to evaluate the facilitators and barriers of work performance and work environment respectively. These factors may have an impact on job tenure and can inform occupational therapy practice. Methods: This study analysed retrospective data of 85 clients who attended a 3-month Employment Internship Program from August 2016 to August 2017. Scores from Work Behaviour Inventory and Work Environment Impact Scale were analysed for associations with job tenure. Repeated measures were used to determine significant changes in Work Behaviour Inventory composite scores across the 3 months. Results: One-month Work Behaviour Inventory composite scores, three Work Behaviour Inventory domains (cooperativeness, work habits, work quality), and three Work Environment Impact Scale domains (time demands, supervisor interaction, architecture) were significantly associated with job tenure. Significant differences in mean job tenure were also found between participants of different internship status. However, these factors did not predict job tenure in regression analysis. There were significant improvements in Work behaviour Inventory composite scores from the first to third month. Conclusions: Work behaviours such as cooperativeness, work habits, and work quality as well as work characteristics such as time demands, supervisor interaction, and workplace architecture may play a role in influencing job tenure. Occupational therapists may consider such factors and provide more targeted interventions to effectively sustain employment.