Leesa Harwood, director of community lifesaving and fundraising at the RNLI, is to stand down in October after more than nine years with the charity.
Harwood said in a statement that commitments outside work meant she could no longer give the time and energy that the role demanded.
She joined the charity in April 2009 as head of engagement and was promoted to her current role in October 2014, according to her LinkedIn profile.
"My time here has been rewarding, humbling and fulfilling," she said. "It has been an absolute privilege to work with such an incredible team of staff and volunteers here at the RNLI and to have been able to add value to this very special charity."
Paul Boissier, chief executive of the RNLI, said Harwood had been "instrumental in some brave decisions that have set us up for success in the 21st century".
In 2015, the RNLI was the first charity to say it would be moving to an opt-in only method of communication with its supporters, a move it anticipated would cost it £36m in income over the following five years.
The policy came into effect on 1 January 2017 and the charity has not yet filed accounts for 2017, so the financial implications of the decision cannot yet be assessed.
But the charity said in February this year that more than 500,000 people had signed up to receive communications from the charity, more than twice the number it had originally anticipated would do so.