Pennell grew up in Richland, Washington, where her father worked at Pacific Northwest National Laboratory and her mother was a math teacher at Columbia Basin College.

[2] Pennell earned a Bachelor of Arts, summa cum laude, from the University of Washington in 1993 and a Juris Doctor from Stanford Law School in 1996.

From 1999 to 2000, she was a Skadden Fellow at TeamChild, an organization that provides legal services to youths in Yakima, Washington.

President Biden nominated Pennell to the seat vacated by Judge Salvador Mendoza Jr., who was elevated to the United States Court of Appeals for the Ninth Circuit on September 16, 2022.

[8] On November 19, 2024, the United States Senate invoked cloture on her nomination by a 50–42 vote.