Newton married English writer and director Ol Parker in 1997. The couple have two daughters: Ripley, born in 2000, and Nico, born in 2004. Her daughters were named after the character Ripley in the Alien films and the cult singer of The Velvet Underground fame.

In 2006, she contributed a foreword to We Wish: Hopes and Dreams of Cornwall's Children, a book of children's writing published in aid of the NSPCC. In it, she writes vividly about her childhood memories of growing up in Cornwall and the way in which the county's vibrant cultural heritage made it easy for her to "enrich every situation with layers of magic and meaning".

Newton swapped her BMW X5 for a Toyota Prius after protesters bombarded the car with eggs at the gates of her daughters’ school. She then wrote to her celebrity friends, asking them to join her in switching to more environmentally sound cars.