Bentley, who joined Girlguiding UK on 5 November, has also been chief executive of the personal safety charity the Suzy Lamplugh Trust.
She said her first priority would be to increase Girlguiding’s membership and carry out a wide-reaching review of its activities so that it can continue to respond to the needs of girls.
The charity has more than half a million members and volunteers and had an income of £16.8m in 2011.
"I am incredibly proud to be joining an organisation dedicated to helping girls become confident, happy, successful people who realise their ambitions and contribute positively to the world around them," said Bentley.
"Girls today face real and unique challenges as they grow up. I want our programme to continue responding to those challenges, so that we always offer girls what they really need."