His notable supporting roles in films include Reality Bites (1994), Out of Sight (1998), Forces of Nature (1999), Employee of the Month (2004), the Stuart Little film series (1999–2002), Riding in Cars with Boys (2001), Chicken Little (2005), Sahara (2005), Rescue Dawn (2006), Dallas Buyers Club (2013), The Good Dinosaur (2015), Captain Fantastic (2016) and War for the Planet of the Apes (2017).

[2][4] Zahn graduated from Robbinsdale Cooper High School in New Hope in 1986, planning to eventually join the United States Marine Corps.

[2] Zahn attended Gustavus Adolphus College for one semester but dropped out after seeing the original West End production of Les Misérables.

[6] In 1991, Zahn made his professional stage debut in a Minnesota production of Neil Simon's Biloxi Blues after falsely claiming to be a member of Actors' Equity.

[8] In 1991, Zahn formed the Malaparte theater company with a group of actor friends, including Ethan Hawke and Robert Sean Leonard.

He received Oscar buzz for his role as Drew Barrymore's deadbeat ex in Riding in Cars with Boys (2001), and played investigative journalist Adam Penenberg in Shattered Glass (2003).

He researched the role by watching chimpanzee videos on YouTube, and later said that the motion capture process and lengthy digital takes made Bad Ape "the most challenging acting job I've ever had".

[21] Zahn is a lifelong military history buff and has said that one of his biggest regrets was having turned down a role in the HBO miniseries Band of Brothers.