Weiss worked as personal assistant on films such as The Viking Sagas for New Line Cinema. For a brief period, Weiss also worked as a personal assistant for musician Glenn Frey. Weiss went to Dublin in 1995 to study Anglo-Irish literature and met David Benioff, the Screenwriter of Troy. Three years later they met again in Santa Monica, California around 1998.
Weiss's 2003 debut novel, Lucky Wander Boy, is themed around video games. In 2006, Weiss said he had a second novel finished that "needs a second draft". That same year, Weiss completed a screenplay for a film adaptation of the video game series Halo, based on a script written by Alex Garland. However, Director Neill Blomkamp declared the project dead in late 2007.
Weiss also worked on a script for a prequel to I Am Legend. However, in May 2011, Director Francis Lawrence stated that he did not think the prequel was ever going to happen.
On July 19, 2017, Weiss announced that he and Benioff are going to begin production on another HBO series, titled Confederate, after the final season of Game of Thrones. Weiss and Benioff said, "We have discussed Confederate for years, originally as a concept for a feature film - But our experience on Thrones has convinced us that no one provides a bigger, better storytelling canvas than HBO."
On February 6, 2018, Disney announced that both Weiss and Benioff will write and produce a new series of Star Wars films after the finale season of Game of Thrones is completed in 2019.
