As the Scottish co-ordinator for the World Society for the Protection of Animals, she was often faced with heart-breaking scenes of animals that had suffered barbaric cruelty; bears with half their faces missing after being shot during the hunting season in North America, others housed in tiny cages - their stomachs milked for bile through a wound that's never allowed to heal.
She's seen others forced to dance and perform for the amusement of spectators.
"I think what broke my heart more than anything is that the bear is such a majestic creature, but they can be reduced to such a miserable, pathetic-looking thing if they are abused too much," says the 46 year old from Tranent.
"I think bears are one of the most persecuted animals in the world.
"Every species of bear is abused in some way or another, and it seems to happen in lots of different ways - bear farming or baiting or (they're] used in circuses or used for their fur or body parts."
She devoted her time and effort to fighting to protect all kinds of bears, raising awareness and pushing for action to stop the cruelty.
Five years ago, however, it seemed she'd lost her fight when the WSPA decided to relocate its operations and she was made redundant.
"It was a blow," she says. "But I never lost my interest in animal welfare.
"I had to find another job, re-train as a driving instructor, set up another business. But I never stopped fundraising for bears during that time."
But a desire to make a lasting difference for the welfare of the animals she loves kept gnawing away at her.
Last year, as the rest of the country was pulling itself up by its bootstraps after being hit by the recession, Lesley decided to launch her own organisation to do fundraising work for bears.
In October, the Winton Foundation for the Welfare of Bears was launched.
"I wanted to try to do something to bring everything together to work for every species of bear," she says.
"Some organisations deal with specific issues for one species, but I felt it would be nice to do something independent of other organisations so we could do something for all species of bears."
Within months her organisation was able to make its first donation - 700 - to a bear sanctuary in Pakistan run by the WSPA.
That charity was chosen because its efforts are focused on rehabilitating animals who have been used in bear baiting, a horrific practice which, although banned in Pakistan, still takes place in some areas.
The bears, teeth and claws removed, are tethered to a post, where they're set upon by fighting dogs up to six times a day.
The dogs are trained to attack the bear's muzzle and ears, and spectators place bets on which species will come out victorious.
Lesley says the bears are left traumatised and it's only once they're rescued by animal welfare charities that their injuries sustained in the bear baiting events are tended to.
"It's a lifetime commitment, as these bears can't be released back into the wild," she says.
"They have no natural defence, so once the bears are rescued they have to be cared for. We want to try to help with the ongoing care for these bears, to cover the cost of their food and veterinary treatment."
But while the injuries sustained by those bears is instantly obvious, less so is the trauma and pain endured by those that are forced to "dance" and entertain tourists.
These bears face a lifetime of physical and mental distress, usually from the moment they are captured as cubs after their mother has been shot. From then they have a ring forced through their nose and a rope pulled through a hole in the upper palate.
Tugging on the rope causes the bear pain, and the bears are then beaten on the legs and buttocks while music plays - resulting in a diabolical bear "dance'"
The practice has been eradicated in Greece, Turkey and Bulgaria, but still exists in India, Pakistan and Nepal.
Lesley has now set her sights on highlighting the plight faced by bears around the world, and earlier this month the Winton Foundation started its latest fundraising drive - dubbed "Put a Pound in a Pot" - for International Save Bears Day this Monday.
The foundation wants donors to put a 1 in a pot for 21 days from between the February 1 and 21, with the hope of collecting more than 7000.
It's already got some projects in mind for the next round of donations, including money for a bear sanctuary that cares for retired dancing bears, which are often left blind from malnutrition or from brutal beatings.
The extra attention and care required to nurse them back to health is, unsurprisingly, expensive.
Further down the line it hopes to send money to China, to help with efforts to close down farms that collect bile from bears' stomachs for use in Chinese medicine.
But not all bear-related stories are sad ones, as Lesley found out during a trip to a bear rehabilitation centre in Idaho in the United States. Every year hundreds of cubs are orphaned there when their mothers are shot during bear hunting season, which falls between August and July.
Without the care and protection of a mother, the baby bears would normally die in the wild, so they are brought to the Black Bear Rehabilitation Centre.
Lesley says: "They can come in from real babies who haven't even opened their eyes, to slightly older cubs. But they're cared for by (the workers] who basically act as mother bear until they're old enough to be put back in the wild.
"It really is an incredible campaign and a lot of people said bears raised in captivity couldn't be put back into the wild, but the project's been so successful that it's proved that this can happen.
"The project is amazing and I was very, very lucky to be part of the release of 21 of these bears back in to the wild in 2008. It was an absolutely incredible experience - one I'll never forget."
Although running the foundation from her home in Tranent in the little spare time that she has, the animal lover is not alone in her fight to save the bears from persecution. "I had a group volunteers and fundraisers when I worked at the WSPA, and a lot of them are now on board, we've got a lot of people supporting us," she says.
"I think people are becoming more aware of the environment and environmental issues, as well as issues of animal cruelty."
Lesley hopes the Foundation will be able to tap into some of that goodwill, and raise awareness of the different threats facing bears throughout the world, starting with issues that she considers to cause immense cruelty to the animals, such as bear baiting.
But it's not just going to stop there. "I'm looking at lobbying in future. Just because we're a smaller organisation, it doesn't mean that we can't be as successful in others," Lesley declares.
"I'm hoping to set up campaigning and fundraising arms as well, even if it's through letter-writing we can hopefully do our bit and play our part in creating a better world for bears - a world where they're not abused."
What you can do to help
Contact The Winton Foundation for the Welfare of Bears at wintonbearfoundation.org to make a donation
Organise your own fundraising event for the Foundation.
If you don't have money to spare, donate your skills by volunteering for the Foundation
Speak to your local shops, garages or community centres to see if they will take a collecting can.
Save loose change - you'd be surprised how it mounts up.
Spread the word - tell friends and family about the plight of bears and the work of the Foundation.