Sullivan was born in Cardiff, Wales, to Londoner Frederick Anderson (1900–1993), an electrician, and Phyllis (1905–2009), a short story writer.
She has written a number of popular books, mostly suitable for 7–12 year olds, with Welsh themes incorporated into their story lines.
She frequently visits primary schools in Wales to open events and carry out book signings.
In 1993, Sullivan was awarded the Irma Chilton Bursary, which is presented to aspiring children's novelists by the Welsh Arts Council.
[1] Tiron's Secret Journal won the 2006 Tir na n-Og Award for the best English-language book of the year with a Welsh background.