Preheat oven to 350 degrees and lightly oil or butter a 9x5" metal loaf pan.
Put cake mix in a large bowl and stir in mashed banana, oil, and eggs until just combined, taking care not to over-mix.
Pour batter into prepared loaf pan.
Bake for 44-48 minutes, until edges are turning golden and a toothpick inserted into center comes out clean. Let cool in pan for a few minutes before turning out onto a wire rack to cool completely.
Slice and enjoy.