It comes as Pretty Little Liars: Original Sin, which he exec produces and writes on, launched on HBO Max on Thursday.
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The multi-year pact marks Grassi’s second overall deal with the studio and keeps him in business with the company that he first started working with since 2015, when he joined the creative team of Supergirl.
Grassi will develop original series for the studio under the exclusive agreement.
He has worked on Pretty Little Liars: Original Sin with Roberto Aguirre-Sacasa, who Grassi had previously teamed with as an exec producer and writer on The CW’s Riverdale as well as spinoff Katy Keene. He was also a writer and producer on the first season of Supergirl.
Before working with Warner Bros. TV, Grassi was a writer and exec producer on Canadian drama Lost Girl and was a story editor on the first season of Schitt’s Creek, having got his start on Degrassi: The Next Generation.
Grassi said, “It is a thrill for me to call Warner Bros. Discovery home. I’m excited and grateful to be a part of this new chapter, and honored to continue telling stories with Channing Dungey, Clancy Collins White, and the entire Warner Bros. Television Group.”
Grassi is represented by TFC Management, WME, and attorney Cheryl Snow at Gang, Tyre, Ramer, Brown and Passman.