There had been speculation that Foss could be in line for a demotion as part of English's reshuffle.
Foss, MP for Tukituki with a majority of 6490 in 2014, said recent events had led him to reflect on his role as a Minister and MP.
He had spoken to English this week about his future. Foss said the new Prime Minister "plays his cards pretty close to his chest", but he had not been given a message that he would lose his place in Cabinet.
"It is just having an honest discussion with your boss...I was edging this way anyway. He expressed confidence in what I have done and my abilities but at the end of the day there is only room for 20 in Cabinet, and so many outside Cabinet.
"There's no axe falling on me...who knows what could have played out next week. I don't know."
He made his decision to quit yesterday.
"My mind was drifting too often out of this place, and that's not healthy for anybody, so it is time to make the call I did...I found myself reflecting more and more as to what could be outside of Parliament."
English said Foss had been very competent and successful. Asked if Foss would have been demoted had he stayed on, the Prime Minister said he was not far enough through the process to say.
"He was a perfectly competent minister and I enjoyed working with him."
English said he had a "few more discussions to have" but the reshuffle was coming together well.
Foss said he would step down as Minister when the new Cabinet is sworn in, but will continue as an MP for as long as necessary to avoid a by-election.
Asked what he would do next, the former investment banker told reporters that he would ponder that question on the beach over summer "with a couple of beers."
Foss said he was proud of his ministerial achievements, including "world's-best" financial markets and consumer legislation, and leading Statistics NZ through a process of modernisation.
"The new leadership team of Bill English and Paula Bennett has my complete support and I wish them well."
There are now three vacancies in Cabinet, following the departure of John Key, Lotu-Iiga and Foss. A number of backbenchers are agitating for promotion.
