The dog had arthritis, as well as torn CCL ligaments in both knees, meaning her back legs were painful and she was unstable on her feet.
Little Lu-Seal now has her own Instagram page, which boasts over 800 followers and her new owner regularly updates her followers with the chihuahua's progress.
Owner Julia has written about Lu-Seal's weightloss mission for Pet Insider and says a vet can determine how much your dog should be eating based on their weight, age and activity level.
Lu-Seal's vet said she should be consuming 250 calories, which meant Julia could feed her 200 calories in regular food and spare 50 calories for treats.
The chihuahua, who was eight pounds overweight to begin with, could only walk a few feet at a time but can now manage to walk a whole mile.
Having used a fitness tracker to keep abreast of her progress, Julia says Lu-Seal has gone from being in the bottom 20 per cent of her breed to an average chihuahua.
She says lots of obese dogs have joint pain, which prevents them from getting much exercise and her pet chihuahua is now on medication to help her deal with the pain and inflammation from arthritis.
Julia writes: "Weightloss in dogs, just like in humans, takes time. Your dog should only lose 1-2 per cent of his or her body weight each week.
"Check in with your vet regularly to make sure that your dog isn't losing weight too fast or too slow and to adjust their calorie intake as needed to go at the right pace.
"It's a long and hard road, but it's worth it!"