Birney began her writing career in advertising, for Disneyland, and then scripting trailers and commercials for Walt Disney Pictures releases. When the Disney Channel launched, she wrote for Welcome to Pooh Corner and Dumbo's Circus. She also wrote for Zoobilee Zoo, The Puzzle Place, and various ABC and CBS live-action afterschool specials. Animation credits included Snorks, Doug, Maxie's World, Bobby's World, Camp Candy, Adventures from the Book of Virtues, and Madeline.
Since the 1990s, Birney has written children's books featuring Winnie the Pooh, Animaniacs, Chip 'n Dale's Rescue Rangers, Bambi, The Little Mermaid, and Beauty and the Beast. Since 2004, she has written a series of books about Humphrey the hamster, starting with The World According to Humphrey. Other titles include the picture books Pie's in the Oven and Tyrannosaurus Tex and the middle-school books Princess and the Peabodys and The Seven Wonders of Sassafras Springs.
- "Big Trouble for a Little Fraggle"
- "The Great Radish Round Up" (with John Semper & Cynthia Friedlob) and "Lucky Fargy"
- "Where No Fraggle Has Gone Before"
- "Gobo's Song"