The "White Cloud sect" was a name for a heterogeneous group of people doing diverse things under the sect's name: devout laypeople and monks, extortionist elites, and profit-making Tangut monks. At different moments, it was a loose group of local lay practitioners, an official religious institution regulating Buddhist affairs, a business enterprise for networking with rich landowners and high-ranking officials, a social organization that obtained economic privileges for registered groups and individuals, a local entrepreneur that raised small-scale donations, a contractor for printing and selling Buddhist canons, and an advocate of religious syncretism. This dissertation studies the White Cloud movement in Song- and Yuan-era Jiangnan. Analysis of printing work and its relationships with the Mongol colonial regime, Chinese local lay activists, and Tangut diaspora monks reveals the dynamics of local religious activism and how the sect extracted social and financial benefits through religious activities, projects and events in the Hang-Jia-Hu subregion. The Song state banned the sect as a heretical group, but the White Cloud's fortunes changed under the Mongol Yuan state, which recognized it as a new Buddhist school. The movement was mostly led by local laymen rather than monks. Its wealth and reputation peaked with the production of a Buddhist canon during the reign of Khubilai Khan (1276-1294), who provided direct patronage. I examine the Mongol government's political and financial support for this unorthodox religious group - the institutionalization of religious communities (including establishing and abolishing a series of religious bureaus, appointing Tangut clergy officials)--and the response of local activists in turn. The White Cloud movement drew official supervision and attracted foreign monks while maintaining its mostly lay followers and non-doctrinal practice in rural areas. Newly emerging market towns in the Hangzhou-Jiaxing-Huzhou hinterland were the homes of most White Cloud sect devotees and their Buddhist canon's local patrons. The region's canal system fostered close networks for resource exchange. Local labor and material resources were used for the White Cloud sect's Puning canon and the related Tangut script Hexi canon. I emphasize the connections between the two canons and the contributions of Tangut diasporic monks.