Gallner started his career by following his sister along to one of her auditions. He proceeded to guest star on popular shows such as Judging Amy and Close to Home. Gallner joined the cast of Veronica Mars as Cassidy "Beaver" Casablancas toward the end of the show's first season, in May 2005 before becoming a season regular during the show's second season.

After playing Bart Allen in the Smallville episode "Run", Gallner reprised his role on the January 18, 2007 episode "Justice". He also guest starred on Cold Case as a teenage gunman, and recently appeared in the independent film Sublime. He now lives in Los Angeles, California with his girlfriend Jade and with fellow actor Jake Abel.

He guest starred in an episode of Law & Order: Special Victims Unit on October 9, 2007 titled "Impulsive" as a high school student who accuses his teacher (played by Melissa Joan Hart) of statutory rape.

• Jennifer's Body (2009) - Colin Gray
• The Haunting in Connecticut (2008) - Matt Campbell
• Red (2008) - Harold McCormack
• Gardens of the Night (2007) - Ratboy
• Sublime(2007) - Ned
• Danika(2006) - Kurt Merrick
• Red Eye(2005) - Headphone Kid's Brother
• Finding Home(2003) - Young Dave
• Red Betsy(2003) - Charlie
• Wet Hot American Summer(2001) - Bobby's Buddy
• Finding Forrester(2001) - classmate of Jamal