Tierney played Nurse Abby Lockhart on ER, a character that began as a guest appearance as an OB nurse, then expanded in 2000 to a full-time regular part as an ER nurse. Within a year, her work on ER had earned her an Emmy Award nomination, recognition she credits to a "juicy" story arc featuring her and Sally Field. Tierney's character Abby Lockhart graduated from medical school and became a doctor at the end of the 2003–2004 season. Since then, her character has gone on to become the female lead on the show.