Place chicken breasts at the bottom of the slow cooker.
Add carrots, celery, onion, and garlic on top of the chicken.
Pour in the chicken broth and add lemon juice, thyme, salt, and pepper.
Cover and cook on low for 6-8 hours or on high for 4-5 hours.
In the last 30 minutes of cooking, add the spinach.
Before serving, shred the chicken in the soup and stir to combine. Enjoy!