This study examines an elementary school which during enrollment in the European Network of Health Promoting Schools, 1993-2003, and the Norwegian Physical Activity and Healthy Meals Project, 2004-2006, selected physical activity (PA) as a prioritized area. Survey data, school documents, and focus group data were collected and analyzed through a sequential exploratory strategy. After a decade, the students in the upper grades participated in more PA weekly than allocated by national plans, and PA promotion was formalized in curriculum outlines. The findings suggest that motivated teachers were the core of PA promotion. Teacher motivation was linked to an interplay between individual teacher characteristics: positive attitudes, innovativeness, internalization of practice, and teacher skills; and organizational characteristics: leadership, teacher climate, and working strategies. The study highlights the advantages of utilizing a mixed-methods approach to data collection and data analyses. The findings could only be reached by an integrative analysis of data from multiple sources, yielding a more comprehensive and in-depth understanding of the conditions facilitating sustainability of PA promotion in schools as well as other types of policy initiatives. (Contains 7 tables and 1 figure.)