Starting his acting career at age six, Thomas was first seen in the soap The Young and the Restless. His first movie job was starring in Village Of The Damned playing David, one of the possessed children. He then appeared in Star Trek Generations. Later in 1997 Thomas became a regular on the Disney Channel Original Show Honey, I Shrunk the Kids: The TV Show based on the movie of the same name. He played Nick Szalinski on the show for three years. After the show ended in 2000 he went on to appear in many popular television shows including Run of the House, Fillmore!, CSI: Crime Scene Investigation, House, Boston Public, Reba and 7th Heaven. He appeared in movies such as Campus Confidential and An American Tail: The Mystery of the Night Monster. He has won three Young Artist Awards for his work in the The Land Before Time movies and one for his guest appearance on Boston Public. He has also been nominated for a DVD Premiere Award and a Video Premiere Award.

In 2006 Thomas landed a recurring role on NBC's hit show Heroes playing the character of Zach. Allegedly the character was intended to be gay, but was changed at the behest of Dekker or his manager. He played the character for 11 episodes before leaving the show when he won the lead role of John Connor in FOX's new show Terminator: The Sarah Connor Chronicles starring opposite Lena Headey and Summer Glau. The show debuted January 13th, 2008 and continues the story of Sarah and John Connor's lives after the events of the second Terminator film, Terminator 2: Judgment Day. Upcoming projects include the movies Brothers of Blood and From Within.

Thomas was brought up with a musical background, and started writing music at age ten while living in Canada. At the age of fifteen he landed a record deal, however after feeling that he wasn't as involved in the music as he would have liked, he left to concentrate on making his own music. At the age of sixteen, Thomas started writing and producing his own music that infused the drama and musical intircacies of classical with the sound scape and aesthetics of electronica. Thomas describes his unique musical style as "electrofolk". Thomas' first album Psyanotic is complete and is currently in negotiations with record companies. It is expected to be released later this year. The album was completely written, composed, produced, mixed and mastered by Thomas himself. He has also, despite continuing with his busy acting career completed a second album currently titled Protean.

Thomas' mother is a concert pianist. His father is an artist, an opera singer and an actor. As a child he and his parents moved all over the world, spending time in the United States, Europe and Canada. He has a step sister named Diana and a step brother named Erik. Thomas is a vegan and owns two pet pugs named Captain and Noggin and a rabbit named Bella.