Adrian Burder, chief executive of the Dogs Trust, died unexpectedly yesterday after a short illness, the charity has said.
Burder, who was 53, had been at the charity for 24 years, initially serving as a fundraiser and taking over as chief executive in 2014.
Jim Monteith, finance director at the Dogs Trust, who has taken on the role of acting chief executive, said in a statement: "Adrian was one of a kind. His passion for animal welfare changed the lives of millions of dogs in the 24 years he was at Dogs Trust.
"He will be terribly missed by staff, trustees and volunteers past and present and we are so grateful for his immeasurable contribution to making the world a better place for man’s best friend.
"We are all still letting this tragic news sink in. Adrian has been an integral part of our charity for so many years, as a hugely successful fundraiser and then as chief executive."
During the time he oversaw fundraising, Burder grew the charity’s income from £3m to £100m and he was instrumental in the charity’s 2003 rebrand, when it changed its name from the National Canine Defence League, the charity said in a statement.
The Dogs Trust said part of the legacy left by Burder, who was Welsh, would be its new Cardiff rehoming centre, due to open in 2020.
He was also the chair of Dogs Trust Ireland and set up Dogs Trust Worldwide, which awards grants to other organisations and runs conferences to allow the UK Dogs Trust to share its expertise internationally.
Before joining the Dogs Trust, Burder worked for the British Union for the Abolition of Vivisection and the TV licensing department of the Post Office.
He is survived by his wife and two children.