Gather all ingredients and prep by chopping the onion and mincing the garlic.
Heat olive oil in a large heavy-bottomed pot over medium heat until shimmering, then add diced onion.
Sauté the onion for about 5 minutes until it becomes translucent and fragrant, then add the minced garlic and cook for another minute, stirring often.
Add the chicken breasts to the pot and cook for about 7 minutes, turning occasionally, until lightly browned on all sides.
Pour in the chicken broth and stir in the diced tomatoes, cumin, salt, and pepper. Bring the mixture to a gentle simmer and cook uncovered for 20 minutes.
Remove the chicken breasts from the pot and shred them with two forks, then return the shredded chicken to the soup.
Stir in the corn, black beans, and chopped cilantro. Squeeze in the juice of half the lime and stir well.
Let the soup simmer for another 10 minutes, tasting and adjusting seasoning with salt, pepper, and a squeeze of lime for brightness.
Serve hot, garnished with extra cilantro and lime slices, and enjoy a comforting, lively bowl of Mexican chicken soup.