Her parents were businessman Bernt Maartman (1841-1889), whose family came from Bergen, Norway, and Augusta Fahlstedt (1850-1915). Beskow studied Art Education at Konstfack, University College of Arts, Crafts and Design (then called Tekniska skolan, or the Technical school) in Stockholm.

She married in 1897 by former minister and social worker, doctor of theology Natanael Beskow. Elsa Beskow met her future husband at Djursholms samskola while serving was a teacher where he served as head master. They had six sons, including the artist Bo Beskow (1906 - 1989) and geologist Gunnar Beskow (1901 - 1991).

In 1894 Beskow started to contribute to the children's magazine Jultomten. Overall, she would publish some forty books with your own text and image. Beskow frequently combined reality with elements from the fairy tale world. Children meet elves or goblins, and farm animals talk with people. Central themes were the relationships between children and adults and children's independent initiative.

Beskow became one of the most well known of all Swedish children's book artists. Many of her books became classics and are continually reprinted. Beskow also illustrated ABC books and songbooks for Swedish schools. Book pages are often framed by decorative framework of the Art Nouveau style.