John Lewis is searching for a new chairman after Sir Charlie Mayfield said he would step down in 2020.
The departure comes at a challenging time for the partnership, which posted a 99% fall in half-year profits in September.
A replacement for Sir Charlie is not expected to be announced in the second half of 2019.
He said he was giving notice now to enable time for a successor to be recruited.
Sir Charlie said: “Although my departure is still a considerable way off, the appointment of my successor is a key responsibility.
“It is for this reason I have decided to lay out the timetable now to enable an open and thorough process to select the next chairman of the partnership.”
Both internal and external candidates for the job will be considered.
Sir Charlie became chairman of the John Lewis group, which also includes Waitrose supermarkets, in 2007.
He has been at the helm over many years of growth at the business, but more recently it has struggled to cope with rising costs, higher business rates and a weaker pound.
Its most recent financial results which include Waitrose, showed profits for the six months to 28 July fell from £95m for the same period last year to just £1.2m.
At the time Sir Charlie blamed stiff competition, saying it was facing down the most promotional market for nearly a decade.
The department store pledges to be “never knowingly undersold” – cutting prices to match those of competitors, which have also been discounting.
“This year there has been twice as many extravaganza days as there were a year ago and actually the discounts have been even deeper,” he told the BBC in September.
“We’re never knowingly undersold at John Lewis, so of course we are matching that, and that affects margins.”
It was an “extremely valuable” promise, Sir Charlie added.