He attended New York University"s Tisch School of the Arts where he studied film. At this time he met Anna Boden on the set of a student film and, after he finished his thesis short film Struggle, they began dating and decided to collaborate.
He is known for his collaborations with fellow filmmaker Anna Boden. Together they made the short documentaries and Young Rebels before they wrote and he directed the short film Gowanus, Brooklyn, a sample feature aiming to attract potential financiers to their undeveloped script, Half Nelson. The film did not receive adequate financing for years, and while trying to initiate the project, Fleck explains: "We were trying to get it off the ground so we had plenty of time to keep writing and rewriting.
I think the time was valuable because I think we made it as good as we could." In spite of the effort in making the script "as good as we could" prior to filming, Fleck encouraged Half Nelson"s actors to ad lib and improvise, although he and lead actor Ryan Gosling compromised many scenes in the rehearsal process as Fleck thought that Gosling"s ideas were too different from the script in some cases.
He also had several disagreements with Boden while writing. Though he claims their "back and forth" system of rewriting each other"s work has always worked well.
Many other nominations went to Ryan Gosling for his role as Dan Dunne. Fleck and Boden have since co-written and co-directed the 2008 film Sugar, which premiered at the 2008 Sundance Film Festival, about a 19-year-old Dominican who immigrates to the United States to play minor league baseball.
The two wrote the screenplay after researching about many Dominican immigrants who arrive in America to play in minor league towns, saying: "The stories we heard were so fascinating that it became what we were writing before we"d even decided it was our next project".
They also collaborated on a screen adaptation of Ned Vizzini"s young adult novel lieutenant"s Kind of a Funny Story, which was released on October 8, 2010.