Coworking space has flourished in the past decade. Unlike traditional shared services organizations, coworking spaces put a much greater emphasis on ‘sharing’. Members not only can share the physical office space, but also the virtual social spaces created by the coworking space operators managing the office. As coworking spaces provide a community to foster the culture of sharing, which gives rise to social interactions and thus knowledge and idea exchange, entrepreneurs favor such coworking spaces to achieve a higher level of job performance among their workers. Although it is generally accepted that a worker’s job performance varies over time within a job, there have been limited studies on within-person performance sustainability and its comparison with between-person sustainability. We sampled 101 workers of young firms operating in six coworking spaces in Singapore who completed daily surveys twice a day across ten consecutive workdays. By treating participants as the first level and daily observations as the second level, our study develops a dual-path model to explain how daily mutual support influences daily job performance. Our results indicated that daily mutual support is positively related to sustainable job performance after controlling for sleep quality, job requirements and workload stress. Within-person sustainability in mutual support was found to account for part of within-person variance in job performance. We established that mutual support not only predicts job performance, but also varies across workdays. As the collaboration of team members depends on cooperation rather than competition, mutual support is considered essential for team work and thus employees’ job performance. Our study also demonstrated the importance of role breadth self-efficacy as a moderator in the link between mutual support and sustainable job performance. Role breadth self-efficacy refers to the extent to which people feel confident that they are able to carry out a broader and more proactive role, beyond traditional prescribed technical requirements. The results revealed an enhancing moderation effect, where increasing the role breath self-efficacy would enhance the effect of the mutual support predictor on sustainable job performance of workers in young firms operating in the coworking space.