Gallagher was born in College Point, New York, to Elena Gallagher (née Lopez) and Darren James Gallagher. His parents separated when he was a baby and his mother remarried Vincent Casey. Gallagher is of Cuban descent on his mother's side and Irish descent on his father's side. He has four younger half-siblings, Michelle, Kelly, Kyle and Killian. Killian was diagnosed with autism at the age of 3. As a result David is an active supporter and spokesperson for the organization Cure Autism Now.

He graduated from Chaminade College Preparatory School in 2003 and enrolled at the University of Southern California, where he majored in film and television studies, graduating in May 2007.

Gallagher began acting at the age of 2, first modeling for print advertorials in and around New York City which eventually led to commercial work as an actor. He appeared in many television commercials as a toddler for products such as Tyson Foods and Fisher Price. At age 8 he briefly had a recurring role on the soap opera “Loving.”

His film debut came in 1993 when he won the role of Mikey in the sequel “Look Who's Talking Now,” playing the son of John Travolta and Kirstie Alley. In 1995 he played in a production of “A Christmas Carol” on Broadway. He also starred in several TV movies, including “Bermuda Triangle.”

In 1996 he reunited with John Travolta for the movie “Phenomenon.” Later that year, he was cast as Simon Camden in the family drama series “7th Heaven.” “7th Heaven” remained on the air for 11 seasons, making it the longest-running family drama in television history, and also became the highest-rated show on The WB Network.

He filmed a few movies during hiatuses from filming, including the direct-to-video “Richie Rich's Christmas Wish” and “Little Secrets” with Evan Rachel Wood. In 2003, during the eighth season of “7th Heaven,” David left the show in order to attend college full-time. However, he returned to the series part-time during season 9 and for what was thought to be the 10th and final season. In late 2006, The CW unexpectedly picked up the show for an eleventh season. David opted not to renew his contract.

His first role after leaving “7th Heaven” was a guest stint on “Numb3rs,” playing serial killer Buck Winters in September 2006, a role which he reprised in January 2009. He also appeared in the season 6 opener of “CSI: Miami,” playing another suspected killer. He also starred in the horror sequel “Boogeyman 2,” which went straight to DVD. In July 2008, Gallagher guest starred on the TNT drama “Saving Grace” as Paul Shapiro, a troubled young man who was the suspect in a murder investigation. His next guest appearance was in an October 2008 episode of the FOX crime drama “Bones” as Ryan Stephenson. Then in November 2008 he appeared in an episode of the CBS show “Without a Trace” as a suspect named Jeff Ellis who was thought to be responsible for the disappearance of a local nurse.

He's also notable for having voiced Riku in the “Kingdom Hearts” video game franchise and for portraying Seiji Amasawa in the English dub version of Studio Ghibli's “Whisper of the Heart.” He appeared on the second-to-last episode of the series “The Deep End,” where he played a man charged with second-degree murder although he is innocent.

In March 2012, David has been announced to star with Zoe Aggeliki and Jake Weber in “Scared of the Dark,” directed by Takashi Shimizu.