Thomasine Christine Gyllembourg-Ehrensvärd, née Buntzen, was born in Copenhagen, Denmark. At age 16, she married the writer Peter Andreas Heiberg, with whom she had a son, Johan Ludvig Heiberg, also later a writer. In 1800, her husband was exiled for his political activity and she obtained a divorce. The following year, she re-married to Baron Carl Fredrik Ehrensvärd, a Swedish aristocrat, who was himself a political fugitive. He added Gyllembourg to his surname, after his mother.
In 1827, at age 53, she published her first novel, Familien Polonius (The Polonius Family), which appeared anonymously in her son's newspaper, Flyvende Post. It was followed by Den Magiske Nøgle (The Magic Key, 1828) and En Hverdags-Historie (An Everyday Story). The latter work was so successful that she adopted the pen name of "The Author of An Everyday Story" until the end of her life.
She continued to produce popular novels and short stories over the next 20 years. From 1849 to 1851, her collected works were published in a library edition of 12 volumes. She preserved the secret of her authorship throughout her career, even from her closest friends, and it was only after her death that her true identity became known. Her image appeared on the Danish currency in the late 1990s.