Halfpenny's career began in 2004, when she appeared in Casualty episode "What Parents Do", playing an eleven-year-old girl called Karine. She played Lucy Summerbee in the television series Byker Grove from 2004 until 2006.
Since leaving Emmerdale, Halfpenny and her former Emmerdale co-star Jamie Shelton appeared along side each other in Two, a play in which between them they portray all 14 characters. In July 2015, it was announced that Chelsea had joined Casualty as junior doctor Alicia Munroe and would make her first appearance in September. She remained on the show for eight weeks, departing in November. However, she returned to Casualty in July 2016, and currently remains as a regular on the show.