It was revealed last month that the actor would guest star on the show, but the site has now learned that he'll take on a pivotal role.
The Dr. Jekyl and Mr. Hyde-inspired medical drama follows Dr. Jason Cole (Steven Pasquale), a bright neurosurgeon who struggles to contain a dangerous alter-ego, known as "Ian," who threatens to mess with his professional and personal lives.
View slideshow: Cast of NBC's new series 'Do No Harm'
Cromwell will appear in at least three episodes as the lead character's mentor, who or may not have his best interest in mind, according to the show's executive producer, David Schulner.
The veteran actor's other TV credits include "Six Feet Under," "24" and "Boardwalk Empire."
The series also stars Alana de la Garza ("Law & Order"), Phylicia Rasha, Samm Levine ("Freaks and Geeks") and former "Body of Proof" star John Carroll Lynch.