The 34-year-old actress is expecting her second child with her partner, Matte Babel.
The "Dollface" star and "Pretty Little Liars" alum shared the news Monday on Instagram alongside photos of her baby bump.
The actress reflected on "love, life and loss" in the caption after announcing her grandmother's death last week.
"Saying goodbye to a loved one while simultaneously experiencing the joy of welcoming another into this world is the great cycle of life. It is also my most challenging season to date," Mitchell wrote.
"I can't help but think this was the universe's plan all along, knowing I would need other worldly joy to cushion the blow of losing one of the most important people in my life," she said.
"Yet, this is proof that love, life and loss can profoundly exist all at the same time."
"Gram, I miss you every day. Little one, we are so excited to meet you," the star added. "I breathe a sigh of peace knowing you two are already connected in such a cosmic way."
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Mitchell had posted a tribute to her grandmother Jan. 30.
"She was and will forever be my best friend," she wrote.
"From the imaginary games she would play with me when I was little to picking me up from high school because I was being bullied, to keeping scrapbooks of every article and magazine cover I have ever been in, she was my #1 cheerleader."
"My heart is broken."
Mitchell and Babel have been dating since 2017 and already have a 2-year-old daughter, Atlas Noa.
Mitchell is known for playing Emily Fields on "Pretty Little Liars," which had a seven-season run on Freeform from 2010 to 2017.
She now portrays Stella Cole on the Hulu series "Dollface."
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