Shimamoto was born in 1983 in Tokyo, Japan. She made her literary debut in 2001 with her story Shiruetto (Silhouette) while still a student at Tokyo Metropolitan Shinjuku Yamabuki Senior High School, winning the 44th Gunzo Prize for New Writers. Her 2002 novella Little by Little won the 25th Noma Literary New Face Prize and was nominated for an Akutagawa Prize, but did not win. Shimamoto was the youngest person to receive the Noma Literary New Face Prize in its history.
