Clarissa Baldwin, chief executive of the Dogs Trust, is to stand down after 40 years at the charity.
Baldwin, who joined the dog welfare charity in 1974 as head of public relations and became chief executive in 1986, will leave at the end of October.
She will be replaced by Adrian Burder, the charity’s deputy chief executive and marketing director, who the charity said had played a central role in increasing its annual income from £3m to more than £75m since he joined in 1994.
In a statement, the charity praised Baldwin’s "passion, dedication and determination to improve dog welfare" and said she came up with the slogan "A dog is for life, not just for Christmas", which remains one of its key marketing messages 35 years later.
Baldwin, who was awarded an OBE in 2003 for services to dog welfare, said she would leave with a heavy heart.
"I joined Dogs Trust in 1974 with a six-month contract and ended up staying for 40 years," she said. "It has been such a very great privilege to have held this role for such a long time and to have been entrusted with the charity’s growth and development."
Under her leadership, the charity starting working in 10 countries around the world, including Bosnia, Thailand and the Philippines, the charity said. Baldwin also led its move from being a rehoming charity to one that also works in the community and with government on animal welfare issues.
The Dogs Trust said it planned to step up its campaigning work this year and has appointed David Cowdrey, head of communications at the RSPCA, to the new role of campaigns director. He will join in April.