The Rockies invited Casteel to major league spring training in 2016.[11] After going 2–for–24 (.083) with 8 strikeouts, he was assigned to Triple–A Albuquerque to begin the season.[12] In 43 games for the Isotopes, he slashed .230/.273/.368 with 3 home runs and 20 RBI. On July 4, 2016, Casteel was released by the Rockies organization.[13]
On July 7, 2016, Casteel signed a minor league contract with the Seattle Mariners organization.[14] He spent the remainder of the year with the Double–A Jackson Generals, hitting .243/.306/.351 with 3 home runs and 28 RBI across 50 contests. Casteel elected free agency following the season on November 7.[15]
On December 16, 2016, Casteel re–signed with the Mariners on a new minor league contract.[16] He spent the 2017 season with the Double–A Arkansas Travelers, playing in 100 games and hitting .272/.323/.429 with 12 home runs and 61 RBI. Casteel elected free agency following the season on November 6, 2017.[17]
On February 7, 2018, Casteel signed with the Lancaster Barnstormers of the Atlantic League of Professional Baseball[18] He played in 104 games for Lancaster, batting .321/.383/.523 with 16 home runs and 70 RBI. Casteel was named an All–Star for the team, and was also named the Barnstormers Player of the Year for the season.[19] He became a free agent following the 2018 season.
On January 25, 2019, Casteel signed a minor league contract with the Atlanta Braves organization.[20] He played in 118 games for the Double–A Mississippi Braves, batting .263/.334/.477 with 21 home runs and 73 RBI.[21] Casteel did not play in a game in 2020 due to the cancellation of the minor league season because of the COVID-19 pandemic.[22] He became a free agent on November 2, 2020.[23]
On March 14, 2021, Casteel re-signed with the Braves organization on a minor league contract.[24] He was assigned to the Braves' Triple–A affiliate, the Gwinnett Stripers, for whom he played in 68 games and hit .224/.299/.417 with 8 home runs and 28 RBI.[25] He elected free agency following the season on November 7.[26] On November 29, he again re–signed with Atlanta on a minor league deal.[27] Casteel played in 69 games for Gwinnett in 2022, batting .217/.303/.481 with 17 home runs and 47 RBI.[28]
On November 1, 2022, Casteel again re–signed with the Braves on a new minor league contract.[29] He played in only one game for Triple–A Gwinnett in 2023. In 2024, Casteel made 14 appearances, hitting .213/.245/.362 with two home runs and six RBI. On July 19, 2024, it was announced that Casteel had retired. He was placed on the Voluntarily Retired List by the Gwinnett Stripers.[30]
^Tyler Maun (April 14, 2014). "Casteel named Helms winner at BADAs". Australian Baseball League. Archived from the original on October 28, 2014. Retrieved October 17, 2014. Casteel named Helms winner at BADAs
