[1][2][3] He, along with Roger Dingledine, began working on onion routing shortly after they graduated from Massachusetts Institute of Technology (MIT) in the early 2000s.

[1] Mathewson and Dingledine were the focus of increased media attention after the leak of NSA's highly classified documents by Edward Snowden, and the subsequent public disclosure of the operation of XKeyscore, which targeted one of The Tor Project's onion servers along with Mixminion remailer which are both run at MIT.

[6] Tor was developed by Mathewson, along with his two colleagues, under a contract from the United States Naval Research Laboratory.

[6] He is also the primary maintainer for libevent, an event notification library used by some prominent applications like Google Chrome, Transmission and also Tor.

[8][6] Mathewson, along with the other two developers of the Tor Project (Roger Dingledine and Paul Syverson), were recognized in 2012, by Foreign Policy magazine as #78 in their list of the top 100 global thinkers of the year.