He is best known, together with Bryan Konietzko, as the co-creator of the animated TV series Avatar: The Last Airbender and The Legend of Korra, both on Nickelodeon.

Before Avatar, DiMartino worked for twelve years at Film Roman, helping to direct King of the Hill, Family Guy and Mission Hill, in addition to his own animated short, Atomic Love, which was screened at a number of high-profile film festivals.

In a 2010 interview the president of Nickelodeon, Cyma Zarghami, confirmed that DiMartino and Konietzko were developing a new series for the network, called The Legend of Korra.

In September 2018, it was announced that Konietzko and DiMartino would serve as executive producers and showrunners for Netflix's live-action adaptation series of Avatar: The Last Airbender.

[3] On August 12, 2020, Konietzko and DiMartino revealed on social media that they had both departed the show due to creative differences with the Netflix team.