[17] In July 2000, building on dispatching[18] 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.

[17] When he first saw implementations of instant messaging, Dorsey wondered whether the software's user status output could be shared easily among friends.

[18] Dorsey and Biz Stone decided that SMS text suited the status-message idea, and built a prototype of Twitter in about two weeks.

[18] The idea attracted many users at Odeo and investment from Evan Williams,[18] a co-founder of that firm in 2005 who had left Google after selling Pyra Labs and Blogger.

[21] As the service began to grow in popularity, Dorsey chose the improvement of uptime as top priority,[22] even over creating revenue—which, as of 2008, Twitter was not designed to earn.

[25][26] During his time as chairman, Dorsey joined several State Department delegations, including a trip to Iraq in April 2009, led by Jared Cohen.

Dorsey responded to a request from Cohen to delay the maintenance so that it would not affect the revolution in Iran, because Iranians were using Twitter to communicate and coordinate.

"[42] In February 2017, Dorsey and Executive Chairman Omid Kordestani matched a $530,000 donation to the American Civil Liberties Union (ACLU) raised by Twitter staffers.

[45] In September 2018, Dorsey testified before the Senate Intelligence Committee alongside Facebook COO Sheryl Sandberg about the use of social media by Russia and others to meddle in the 2016 presidential election.

[47] A week earlier, Dorsey took part in a TED talk about the social media platform's spread of abuse and misinformation, which has brought him criticism.

[48] On August 30, 2019, Dorsey's personal Twitter account was allegedly breached for nearly an hour by a group calling itself the Chuckling Squad, posting and retweeting numerous racist tweets.

Republican Roger Wicker, who chaired the committee, led the charge to force the CEOs of Twitter, Facebook and Google to testify about the legal immunity the tech platforms receive under Section 230 of the Communications Act of 1996.

[62] Dorsey, along with co-founder Jim McKelvey, developed a small business platform to accept debit and credit card payments on a mobile device called Square, released in May 2010.

Square is also a system for sending paperless receipts via text message or email, and is available as a free app for iOS and Android OS.

[72] In 2020, for months, Square began withholding up to 30 percent of the funds that merchants collected from customers using its Cash App.

[81] In a 2024 interview, Dorsey said that Bluesky's shift toward a traditional corporate structure and the introduction of centralized moderation tools were major factors behind his leaving the company.

[86] In January 2018, it was reported that Dorsey would not seek reelection at Disney's March annual meeting, due to increased difficulty with conflicts of interest.

[98] In 2019, Dorsey contributed financially to the campaigns of 2020 Democratic presidential candidates Tulsi Gabbard (now Republican) and Andrew Yang.

[100] Until 2021, Dorsey applied "world leader" exceptions that enabled President Donald Trump to post content on Twitter that would normally be removed or generate sanctions per the platform's rules.

[101] On January 6, 2021, after pro-Trump supporters stormed the U.S. Capitol, Twitter applied a 12-hour timeout to Trump's account for violating its Civic Integrity policy.

[104] In June 2023, Dorsey endorsed Robert F. Kennedy Jr. in his campaign to become the Democratic party nominee in the 2024 United States presidential election.

[108] In October 2019, Dorsey donated $350,000 to #TeamTrees, a nonprofit started by YouTubers MrBeast and Mark Rober, that pledged to plant 20 million trees by the end of 2019.

[109][110] On April 7, 2020, Dorsey announced that he would move about $1 billion of his equity in Square, Inc., just under a third of his total wealth, to Start Small, LLC, and to relief programs related to the coronavirus.

[115] In August 2020, Dorsey donated $10 million to Boston University's Center for Antiracist Research, founded by Ibram X.