Fenn studied linguistics and astronomy at the University of Hertfordshire, where she became the president of PSiFA (the local student Science Fiction society) from 1984-1985. She also helped organised Shoestringcons and edited their newsletter/fanzine, Hypo-Space, for a period.
Fenn has had several short stories published in various magazines and anthologies. Her debut novel, Principles of Angels was published in June 2008 (UK) and is the first of the Hidden Empire series. Her second novel, Consorts of Heaven, published in June 2009, mixed elements of fantasy with science fiction and was not a direct sequel. The third book, Guardians of Paradise (2010) brings together threads from the first two and the fourth and fifth, Bringer of Light (2011) and Queen of Nowhere (2013) take the story in a new direction. In 2012, Fenn published the short story collection Downside Girls. It features interlinked stories set in the universe of her Hidden Empire series.
Fenn was guest of honour at Novacon 42 in November 2012.