Heat a gas grill to high heat, approximately 450 degrees.
Remove the chicken from the packaging and pat dry with paper towels. Place the chicken between 2 pieces of plastic wrap and use a meat mallet to gently pound the chicken to an even 1/2-inch thickness.
Brush the breasts on both sides with the olive oil and season with the salt and black pepper.
Cook the chicken on grill for 4 to 5 minutes on the first side, until golden brown and slightly charred. Flip and continue grilling until just cooked through, 3 to 4 more minutes, or until an instant read thermometer inserted in the center reads 155 degrees.
Transfer the chicken to a cutting board and tent with aluminum foil for 5-10 minutes, then cut into bite size pieces.
Meanwhile, in a large salad bowl gently toss the salad greens with English cucumber, red bell pepper, red onion slices, and diced mango.
Add the chopped chicken and toss again. Serve drizzled with mango dressing.