In 2009, Schmidt was cast as the lead of the TV show Big Time Rush. Kendall portrays a character like himself with the name of Kendall Knight, the calm and relaxed member of Big Time Rush. He also was a member of boy band Big Time Rush. Nickelodeon signed Big Time Rush and the debut album, titled BTR, was released on October 11, 2010 and debuted at number 3 on the Billboard 200. Later was certified Gold for shipments of 500,000 copies in the U.S. In November 2010, it was announced that a Christmas special of Big Time Rush would debut later that month, and that a Christmas EP would be released to coincide with the episode. On November 30, 2010, they released the Holiday EP Holiday Bundle, with two songs: "Beautiful Christmas" and the cover of "All I Want for Christmas is You", originally performed by Mariah Carey. On February 15, 2011, "Boyfriend" was released as the band's first official single to mainstream US radio and peaked at number seventy-two on the Billboard Hot 100, becoming their most successful song to date.
Schmidt and the group announced they would be recording their second studio album, just after Nickelodeon renewed the series for a third season. Their second album, Elevate, was released on November 21, 2011 and debuted at number 12 on the Billboard 200, selling over 70,000 copies in its first week. Though for a lower peaking than their previous, the album sold more copies than the previous album. The first single, "Music Sounds Better with U", was released on November 1, 2011. He announced would be starring in their full-length 2012 film, Big Time Movie. In 2013 Nickelodeon renewed the Big Time Rush series for a 13-episode fourth season, production started on January 7. The third album, 24/Seven, was released on June 11, 2013 and many critics to draw comparisons between the band's third album and NSYNC's third album Celebrity. The band went on hiatus in March, 2014.
In 2014, it was announced that Schmidt will be making solo music. He went on tour with his band, Heffron Drive. The tour was called the Heffron Drive Winter Tour. A 2015 tour that will take place around the world is being planned. Heffron Drive released their first single, "Parallel", on March 25, 2014, and their first album, Happy Mistakes, later that same year. The band made an acoustic version of the album, titled Happy Mistakes Unplugged. In 2015, Schmidt was featured on a Hilary Duff track called "Night Like This" from her new album Breathe In. Breathe Out.. As of September 2015, Schmidt has been dating Brenda Rodriguez.