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 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 the role on the January 18, 2007, episode "Justice" and the season eight finale. He also guest starred on "Cold Case" as a teenaged gunman in the episode "Rampage", and appeared in the independent film "Sublime". In 2007, he guest starred in an episode of "" titled "Impulsive" as a high school student who accuses his teacher (played by Melissa Joan Hart) of statutory rape. He also guest starred in an episode of "Bones" as Jeremy Farrell, the suspected murderer of a child beauty-pageant contestant. He also played the lead role in the horror film "The Haunting in Connecticut", playing a cancer patient who is tormented by evil spirits.