McCartan starred as Brad Majors in the 2016 Fox musical television film The Rocky Horror Picture Show: Let's Do the Time Warp Again.
In July 2013, McCartan began playing the recurring role of Diggie on the Disney Channel comedy Liv and Maddie.
[14] In 2016 McCartan was cast in the lead role of Brad Majors in the 2016 Fox musical television film The Rocky Horror Picture Show: Let's Do the Time Warp Again.
[18] In 2019, he starred in the role of Mac in the Roundabout Theatre Company's new musical Scotland PA.[19] In November 2019 he made his solo show debut at Feinstein's/54 Below.
[20] He joined the cast of Frozen on Broadway as Hans in February 2020 opposite McKenzie Kurtz as Anna and Ciara Renée as Elsa.
[30] In 2024, McCartan was announced to star in the Ahmanson Theatre and Broadway productions of Stephen Sondheim's Old Friends from February to June 2025.
[32][33][34] McCartan began dating actress and singer Dove Cameron who was his Liv & Maddie co-star in August 2013.
[14] In October 2017, McCartan began dating Samantha Fekete, with whom he ran a YouTube channel called The Stage Door, previously known as Sam & Ryan.
[37] In September 2023, McCartan and Fekete revealed that they had amicably ended their romantic relationship, but planned to continue running The Stage Door together and remain friends.