This paper presents the first large‐scale investigation of the users and uses of WorldCat.org, the world's largest bibliographic database and global union catalog. Using a mixed‐methods approach involving focus group interviews with 120 participants, an online survey with 2,918 responses, and an analysis of transaction logs of approximately 15 million sessions from WorldCat.org, the study provides a new understanding of the context for global union catalog use. We find that WorldCat.org is accessed by a diverse population, with the three primary user groups being librarians, students, and academics. Use of the system is found to fall within three broad types of work‐task (professional, academic, and leisure), and we also present an emergent taxonomy of search tasks that encompass known‐item, unknown‐item, and institutional information searches. Our results support the notion that union catalogs are primarily used for known‐item searches, although the volume of traffic to WorldCat.org means that unknown‐item searches nonetheless represent an estimated 250,000 sessions per month. Search engine referrals account for almost half of all traffic, but although WorldCat.org effectively connects users referred from institutional library catalogs to other libraries holding a sought item, users arriving from a search engine are less likely to connect to a library.