Castano spent all of 2016, his first professional season, with Johnson City, posting a 6.19 ERA along with a 2–5 record in 12 games (11 starts).

In 2017, he played for the State College Spikes, where he greatly improved, pitching to a 9–3 record and a 2.57 ERA over 14 starts,[5] earning New York-Penn League All-Star honors.

[6] On December 13, 2017, the Cardinals traded Castano, Sandy Alcántara, Magneuris Sierra and Zac Gallen to the Miami Marlins in exchange for outfielder Marcell Ozuna.

He began the 2018 season with the Jupiter Hammerheads, and also spent time with the Gulf Coast Marlins and the Greensboro Grasshoppers during the year.

Over thirty games (11 starts) between the two clubs, Castano pitched to a 7–4 record with a 3.48 ERA, striking out 104 batters over 119 innings.

On August 8, 2020, Castano made his MLB debut against the New York Mets, giving up 4 earned runs over 4.1 innings.

In 2022, Castano appeared in 10 games for Miami (7 starts), and pitched to a 1–3 record and 4.04 ERA with 20 strikeouts in 35.1 innings of work.