Eating healthier at work can substantially promote health for office workers. However, little has been investigated on designing pervasive health interventions specialized in improving workday eating patterns. This paper presents a design study of an mHealth app called EAT@WORK, which was designed to support office workers in the Netherlands in developing healthy eating behaviors in work routines. Based on semi-structured interviews with 12 office workers from a variety of occupations, we synthesized four key features for EAT@WORK, including supporting easy access to relevant knowledge, assisting goal setting, integrating with health programs, and facilitating peer supports. The user acceptance of EAT@WORK was examined through a within-subject study with 14 office workers, followed by a qualitative study on the applicability of app features to different working contexts. Quantitative results showed that EAT@WORK was experienced more useful than a benchmark app (p < 0.01) and EAT@WORK was also perceived easier to use than the benchmark app (p < 0.01). The qualitative analysis suggested that the goal assistant feature could be valuable for different working contexts, while the integrated health program was considered more suitable for office work than telework. The social and knowledge support were expected to be on-demand features that should loosely be bonded with the working contexts. Based on these findings, we discuss design implications for the future development of such mHealth technologies to promote healthy eating routines among office workers.