Ernie Reyes Jr. made his film debut at age 13 in the 1985 film The Last Dragon. The same year, he shared the screen with Arnold Schwarzenegger and Brigitte Nielsen in Red Sonja. He simultaneously starred in his own television series, Sidekicks, a spin-off of the original Walt Disney one-time special The Last Electric Knight alongside Gil Gerard presented by Michael Eisner. He has appeared in an episode of the hit television series MacGyver in 1988. In 1990, Ernie performed the costumed stunt work for Donatello in the iconic pop culture film Teenage Mutant Ninja Turtles. Following the success of the first film, the production team behind TMNT loved Ernie’s performance so much, they created the lead role of Keno for him in the movie's sequel Teenage Mutant Ninja Turtles: The Secret of the Ooze! Ernie is known for his starring role in Surf Ninjas, which has gained cult movie status over the years. He later appeared in films such as Rush Hour 2, playing Zing and in The Rundown alongside WWE wrestler Dwayne "The Rock" Johnson. Reyes guest starred in three episodes of the TV series Kung Fu: The Legend Continues alongside David Carradine. In 2006, Reyes hosted the MTV martial arts reality show Final Fu.
Ernie Reyes Jr. has a professional fight record of 3–0 in Strikeforce, a large mixed martial arts organization. The group started out as a Muay Thai and kickboxing organization. Ernie fought Anthony Elkaim the WKA ISKA Champion. Reyes knocked out the champion in the 3rd round. His next two fights would be won by decision, including a unanimous decision over Veasna Thach which aired on ESPN. Ernie Reyes Jr.'s fight against the champion Anthony Elkaim also aired on ESPN.
Ernie Reyes Jr. has done motion capture stunts for blockbuster films such as Avatar and Alice in Wonderland. He also played a cemetery warrior in Indiana Jones and the Kingdom of the Crystal Skull who gets killed by his own poison dart. Reyes guest starred on NCIS: Los Angeles "The Frozen Lake" playing "Jay Thapa", a Nepalese soldier who is an expert in knife-fighting. He has also guest starred in NCIS: Los Angeles which included a great fight scene against LL Cool J and Chris O’Donnell.