Dorsey was born and raised in St. Louis, Missouri, the son of Tim and Marcia (née Smith) Dorsey. He is of English and Italian descent. His father worked for a company that developed mass spectrometers and his mother was a homemaker. He was raised Catholic, and his uncle is a Catholic priest in Cincinnati. He attended the Catholic Bishop DuBourg High School. In his younger days, Dorsey worked occasionally as a fashion model. By age 14, Dorsey had become interested in dispatch routing. Some of the open-source software he created in the area of dispatch logistics is still used by taxicab companies. Dorsey enrolled at the University of Missouri–Rolla in 1995 and attended for two-plus years before transferring to New York University in 1997, but he dropped out two years later, one semester short of graduating. He came up with the idea that eventually became Twitter while studying at NYU.
While working on dispatching as a programmer, Dorsey moved to California. In 2000, Dorsey started his company in Oakland to dispatch couriers, taxis, and emergency services from the Web. His other projects and ideas at this time included networks of medical devices and a "frictionless service market". In July 2000, building on dispatching and inspired in part by LiveJournal and by AOL Instant Messenger, he had the idea for a Web-based realtime status/short message communication service.
When he first saw implementations of instant messaging, Dorsey wondered whether the software's user status output could be shared easily among friends. He approached Odeo, which at the time happened to be interested in text messaging. Dorsey and Biz Stone decided that SMS text suited the status-message idea, and built a prototype of Twitter in about two weeks. The idea attracted many users at Odeo and investment from Evan Williams, a co-founder of that firm in 2005 who had left Google after selling Pyra Labs and Blogger.