Before producing and writing for Entourage, Ellin served as a staff writer for Life with Bonnie, which starred Bonnie Hunt. The series ran from 2003-2004. Ellin has also written screenplays for two films, Kissing a Fool and Phat Beach. He moved to Los Angeles in the early 90's to be a stand-up comedian and soon got a job in the mail room at New Line Cinema. It was there where he befriended film producer Mike DeLuca who funded Ellin's first short film The Pitch, starring unknown actor David Schwimmer. That short film got Ellin accepted to the American Film Institute, where he graduated from in 1994. In 1996, he directed the independent comedy film Phat Beach. The film was made on a very low budget, and Ellin has called it "Probably the worst movie ever made". Two years later, Ellin made another independent comedy film Kissing a Fool, starring David Schwimmer, who was now known due to his role on Friends which was in its fourth season.
Kissing a Fool was sold for distribution and Ellin subsequently sold a few screenplays to Miramax. One, The Pledge was never produced although Ellin was paid seven figures for the script and was set to direct. He sold another unproduced screenplay for mid-six figures.
Ellin's real success came in 2004 with Entourage. The HBO show ran for eight seasons and Ellin made an eight-figure pact with HBO to continue producing television.