Bentley, who has been chief executive of Girlguiding since 2012, will succeed Sir Tony Hawkhead, who stood down in February because of health concerns.
Before joining Girlguiding, Bentley was chief executive of the sexual health charity the Family Planning Association, now fpa, and had previously spent almost five years in the same position at the personal safety charity the Suzy Lamplugh Trust.
Bentley, who was named Most Admired Chief Executive at the Third Sector Awards in 2014, is a trustee of the National Council for Voluntary Organisations and was vice chair of the housing and homelessness charity Shelter for five years until 2016.
Bentley said she would be very sad to leave Girlguiding but felt it was the right time to take on a new challenge.
"Action for Children works with some of the most disadvantaged children and young people in our society," she said. "Its work is crucial and it will be an honour to join the trustees, staff and volunteers to take the charity forward."
She will be paid £150,000 a year, the same salary Hawkhead was on, and will take up the new role on 1 August.
The charity had an income of £159.8m in the year to the end of March 2017 and employs more than 5,000 people, according to figures on its entry on the Charity Commission’s online register.
John O’ Brien, chair of Action for Children, said Bentley would bring "much-valued experience and skills" to the role.
"Action for Children is proud to have her on board to help transform the lives of disadvantaged children across the UK," he said. "I know she will help us to make a huge difference to the lives of the children, young people and families we support."
Carol Iddon, managing director of children’s services at Action for Children, who has been acting chief executive since Hawkhead’s departure, will continue to lead the charity until Bentley takes up her new role on 1 August.