Canine crusade: Clarissa Baldwin - CEO of the Dogs Trust - has quite a tail to tell

Baldwin is the brains behind the slogan "A Dog is for Life not just for Christmas"

A slogan is for life: Clarissa Baldwin and friend (Charlie Forgham-Bailey)

When Clarissa Baldwin began work at the Marble Arch offices of the Dogs Trust in 1974, she had her doubts. "It was the kind of place where the photocopier is in the basement and the kettle is on the top floor," she says. "It was so cold we had to wear mittens to type on our steam typewriters."

Having answered an advertisement for a six-month contract because she thought she'd "like do something more useful", Baldwin quickly realised that she was stepping into a different world from the slick environs of her previous PR job. "I arrived in high heels and sat on a sofa that I'm quite sure every stray dog had peed on!" she says, with a peal of giggles.

Baldwin's initial impressions were confounded, and the six-month contract was extended rather longer than predicted. Baldwin has been chief executive of the trust since 1986, and is the brains behind the slogan "A Dog is for Life not just for Christmas". The phrase has just turned 35 and (now trademarked) still has a firm place in the British public's collective consciousness. "We had no money," Baldwin says, "and the trustees sent me away to come up with 'something that encapsulates our work, and is cheap, cheerful, and effective'. So, a bottle of wine with my husband later..."

Baldwin, whose tone has a great deal of jollity for someone who sees animal cruelty daily, says that she loves her job, and claims that getting up on a Monday morning has never been a problem. "There's a lot to get up for... It's life-saving work." And, she says, while there are horror stories, there are also lots of happy endings. These are mostly what she chooses to communicate to the public, in the competitive world of charity fundraising. "We are that sort of warm, friendly, upbeat charity," she says. She focuses on the 18 rehoming centres around the UK, which have brought the number of destroyed dogs down from 46,000 in 1996 to around 8,000 this year.

And yet, to use the inevitable pun, the Dogs Trust has shown itself to have real teeth. "Politically, we've achieved a lot," Baldwin says. The trust was instrumental in getting the Animal Welfare Act passed in 2006, and has been chairing the Microchip Alliance since 2011 — it includes workers from the postal services, no joke — that has resulted in a law requiring all dogs to be microchipped from 2016. And the issue of dangerous dogs is one that Baldwin feels strongly about. "It's desperately unfair that because a dog looks a certain way it is criminalised," she says. The trust has two or three dogs that are "exempted" (banned) and Baldwin says they are "delightful". Of course, she says, you have to be careful, but for that reason she suggests that it is the owners who should have the training.

Dogs Trust's eco-rehoming centres and the Dogmobiles (portable kennels that travel around the country showcasing dogs) are now all the rage, but influencing policy is authentic to the spirit in which the company was founded in 1891. Conceived by a Victorian woman, the early campaigns were against vivisection. In the Second World War, the trust stopped the government from killing strays. In 1956, it protested about animals being sent into space. Baldwin wants to keep challenging. "In 1997, we decided we weren't just a soup kitchen," she says. "We wanted to try to solve problems." Now the trust is keen to find out what happens to greyhounds when they retire (the industry, Baldwin says, is pretty quiet about that), and remind people that, of the eight million or so dogs in the UK, only a handful have "naughty owners".

Is she allowed to say which was her favourite dog of all time? Of course — that would be Kirsty, a little terrier, who is looking down from the happy hunting ground in the sky, undoubtedly after a very good life with the woman who is surely a dog's best friend.