Born in Brisbane, Astley studied arts at the University of Queensland. After marrying Jack Gregson in 1948, she moved to Sydney where she taught at various high schools, as well as kept up with her writing. She tutored at Macquarie University in North Queensland. In the late 1980s they moved to Nowra on the NSW South Coast, and, after her husband's death in 2003, she moved to Byron Bay to be near her only child, Ed Gregson, a musician and television producer.
In addition to her passion for writing, Astley, along with her husband, had a great love of music, particularly jazz and chamber music.
Wyndham writes that "in person and in print, the chain-smoking Astley was unsentimental, wickedly funny and yet had a deep kindness and a loathing of injustice towards Aborigines, underdogs and misfits".
Thea Astley died at the John Flynn Hospital on the Gold Coast in 2004.
In 2005, the Thea Astley lecture was instituted at the Byron Bay Writers Festival delivering the inaugural one.