[1] His work as a programmer includes launching GNU Image Manipulation Program (GIMP) while still in college,[2] and assisting the source code development of CockroachDB, the namesake software of Cockroach Labs.

While still a student in 1995, he developed the first version of GNU Image Manipulation Program (GIMP), along with his roommate Spencer Kimball.

[3][4] Mattis was also a member of a student club at Berkeley called the eXperimental Computing Facility (XCF).

[13] Mattis moved to Square's New York City office where he became a senior member of the company's East Coast team.

[1] He launched CockroachDB as an open source project on GitHub with Kimball, along with ex-Google Reader team member Ben Darnell.

[1] Mattis serves as the company's chief technical officer and also contributes to the source code development of CockroachDB.