Born in Dallas, Texas, and raised in Rowlett, Texas, he currently resides in Rockwall, Texas. He attended Vernon Schrade Middle School as a child. His parents, René & Betsy Castro, are Colombian, and he is the first person in his family to be born in the United States, followed by younger siblings Michael & Jackie. Their father, René Castro, is an architect. Castro plays guitar and grew up playing drums. He did not begin singing until he was in college. Castro attended Rowlett High School and later graduated in 2005. Jason was a part of the National Honor Society and received a score of 1340 on his SATs. He was also a member of the high school soccer team and played defensive midfield. Castro was a junior majoring in construction science at Texas A&M University on a Full-Academic scholarship. He had no formal training before auditioning for American Idol. Castro first appeared on television as the love interest of Cheyenne Kimball on her MTV reality show, Cheyenne.

Castro performed as a drummer with the rock band Charlemagne, which later became Keeping Lions. They once played shows with the likes of local bands, such as Oh, Sleeper.

Castro auditioned for American Idol in Dallas, Texas, singing the Keith Urbansong "Once in a Lifetime". He was not shown on the television series until the finals due to legal restrictions on the songs he chose to perform. He has stated that his musical influences include Ray LaMontagne and Kirk Baxley.[9] He made some appearances on the MTV reality show Cheyenne and was also featured in Cheyenne Kimball's music video "Hanging On".

He was in the bottom 3 only once in "Year they were born" week. He was voted off the competition on May 7, 2008. Castro's rendition of the song "Hallelujah" performed on American Idol, combined with the judges' favorable comments, propelled viewers to the iTunes Store to download the Jeff Buckleycover of the Leonard Cohensong, which propelled it to the #1 spot on iTunes' singles chart.[citation needed] Buckley's version of the song sold 178,000 digital downloads in the week following Castro's performance, to debut at #1 on Billboard's Hot Digital Songs chart. According to the March 22, 2008 issue of Billboard this was the highest boost in digital sales a song had ever received after being performed by a contestant on American Idol. It was the first time Buckley's version of the song charted on a Billboard chart, as well as the first U.S. number one song on any chart for Buckley, who died May 29, 1997 at age 30. Also, Castro's version briefly appeared in the number one spot on the iTunes chart before it was removed due to American Idol not wanting any contesant to be favored.