He first acted as Lucas in Phil Joanou’s film Entropy. He then started his voice role as Wee Willy, the main character in the unaired Nickelodeon/Nick Jr. pitch pilot for the PBS Kids TV series Super Why!.  Eisen later appeared on a 2000 episode of Nick Jr.’s Dora the Explorer as Baby Red Fish and portrayed a boy in Richard Benjamin’s Marci X. He was cast as Pablo in the children’s TV show The Backyardigans for its first season. In the last three seasons of the series, he was replaced by Jake Goldberg. He then provided the voice of Aang in Nickelodeon’s Avatar: The Last Airbender. He voiced Lucas Nickle in the movie The Ant Bully.

He also has done voice work for video games, reprising his role as Lucas Nickle for the video game The Ant Bully and as Aang in the Avatar: The Last Airbender video games, including the series’ titular video game adaptation, Avatar: The Last Airbender – The Burning Earth and Avatar: The Last Airbender – Into the Inferno.