A Positive Work and Organizations (PWO) approach uses scientific methods to improve the understanding of individual, team, and organizational well-being and performance (Warren, et al., 2017). One area of inquiry within the PWO approach involves designing and evaluating positive psychology interventions (PPIs) at work. The current research used a multi-phase exploratory sequential mixed method design to evaluate the effectiveness of PPIs at work using process evaluation, and tested a framework of employee positive functioning expanding on Seligman’s PERMA Theory of Well-Being (Seligman, 2011). Four related studies were carried out examining the fidelity, quality of implementation, and effectiveness of 22 PPIs at work, and testing the validity of a new Employee Positive Functioning (EPF) scale with more than 1,000 full-time employees. The ability of the EPF Scale to predict important work outcomes was also examined. Taken together, these studies show that PPIs implemented at work can be effective at improving employee well-being and organizational performance. They also show that the EPF scale exhibited convergent, discriminant, criterion, predictive, and incremental forms of validity with other well-being and performance measures, as well as measurement invariance across job functions. In addition, the EPF scale was predictive of important work outcomes, such as turnover intentions, job-related affective well-being, and individual, team, and organizational adaptivity, proactivity, and organizational proficiency. It is recommended that organizations consider using the validated EPF scale to determine the specific needs of their workforce, and to use this needs assessment to help tailor positive psychology interventions to be more effective in work settings. The benefits of multicomponent PPIs and the theoretical and practical implications of this study for the design and evaluation of future PPIs at work are discussed.