The presenter announced in April that she will leave the morning programme ahead of its relocation to Salford, Manchester. Williams's co-host Bill Turnbull has chosen to remain in his position.
"Family reasons mean I can't move with the show next year," she told What's On TV. "My son will be in the middle of his A-levels and our elderly relatives live in the South-East.
"It will have been 10 years since I first sat on the Breakfast sofa, but I'm looking forward to trying other things. It also means I won't have to go to bed at 7.30pm every evening anymore!"
Recalling her highlights from working on the programme, she said: "I should say meeting politicians, shouldn't I? I've interviewed three prime ministers. But you know what? It was Hollywood star Will Smith.
"Will was such a charmer. He arrived two hours early and introduced himself to everyone in the studio, and I got to touch his ears! He has very soft ears if you're interested!"
Williams will present new five-part series National Treasures Live alongside Dan Snow from Wednesday, August 10 on BBC One.
> BBC One boss 'not scared about Breakfast move'