The 49-year-old "Cinderella" star will play Georgia, "the beautiful, forthright mother" of two brothers who get a new nanny, said The Hollywood Reporter.
The British culture clash drama about a young nanny from Leicester finding her feet in 1980s London will be Hornby's first drama written for TV.
"Nina Stibbe's book 'Love, Nina' has already established itself as a much-loved piece of comic writing and I love it. Her observations and worldview were the inspiration for a show that we think captures the same spirit.
The show promises "a charming, laugh-out-loud" series that "celebrates family and friendship in all its chaotic glory" and is inspired by Stibbe's book, which consists of the letters she wrote as a young live-in nanny to her sister back home.
The "Love, Nina" cast also includes Faye Marsay ("Game of Thrones") as Nina, Jason Watkins as author Malcolm Tanner and Josh McGuire ("Mr Turner") as Nina's on-off boyfriend.
Ethan Rouse and Harry Webster will play brothers Joe and Max.