Integrating organizationally targeted wellness strategies into the routine conduct of business has shown promise in engaging captive audiences at highest risk of obesity and obesity-related health consequences. This paper presents a process evaluation of the implementation of the University of California, Los Angeles, Working Out Regularly Keeps Individuals Nurtured and Going (WORKING) pilot study. WORKING focuses on integrating physical activity and nutrition practices into workplace routine during non-discretionary paid work time. The purpose of the evaluation was to assess the quality of implementation and to understand factors that facilitated or hindered organizations' full uptake of the intervention. Fifteen worksites were randomly assigned to an intervention condition. Qualitative data were gathered through routine site visits and informant interviews conducted throughout each worksite's intervention period. Worksites were classified into one of four implementation success categories based on their level of adoption and maintenance of core intervention strategies. Six key factors emerged that were related to implementation success: site layout and social climate, wellness infrastructure, number and influence of Program Champions, leadership involvement, site innovation and creativity. This pilot study has informed the conduct of WORKING II; a cluster randomized controlled trial aimed at enrolling 60-70 worksites in Los Angeles County.