[5] She began training in voice, dance and acting at the Annie’s Playhouse School of Performing Arts in Far Hills, New Jersey at the age of six.

[1] She was the American face of Clean and Clear's 2015 Awkward to Awesome campaign,[1] and was the understudy for the role of Louisa von Trapp in the NBC television special The Sound of Music Live!.

[13][14] Donnelly stars as Addison in the Disney Channel Original Movie musical Zombies, which premiered on February 16, 2018.

[22][23][24] In 2022, Donnelly was cast as Mary Campbell in the Supernatural spin-off The Winchesters,[25] which was picked up to series by The CW in May and cancelled after one season.

[28] In 2023, she voiced Kara Zor-El / Supergirl in the film Legion of Super-Heroes, a role she reprised the following year in the Justice League: Crisis on Infinite Earths trilogy.