Pugh, who hasn't announced what she will do when she leaves, will have worked with the children's charity for eight years.
She said she had decided not to extend her contract, as it was the "right time" to move on.
"Eight years is a good time. I think I've made a difference and that things have moved forward a lot. I decided to leave while the charity is in such good shape and I still have a lot of energy left to do other things."
During Pugh's time in charge, the number of families the charity works with has increased from several hundred to 7,000, while turnover has grown from 1.3m to more than 7m.
- Britain's first home for abandoned children, which was opened in 1739 by Coram Family founder Thomas Coram, reopens next week as a museum that will trace the charity's history.
