The major distinction between Emmanual Kant’s deontological reasoning and Mill’s utilitarian reasonsing is that deontological reasoning refers to duty, which is usually determined without regard to circumstances or consequences where as utilitarian reasoning always considers circumstances and consequences.
A good case can be made that classic deontological theories, like Kan't categorical imperative, are just utilitarian theories very well disguised.
Utilitarianism: Utilitarianism is an ethical system that is most often attributed to philosophers such as John Stuart Mill and Jeremy Bentham. Utilitarianism believes that the most ethical thing to do is to maximize the happiness within a society. Utilitarian’s believe that actions have calculable outcomes and that ethical choices have outcomes which lead to the most happiness to the most members of a society. Utilitarianism is often considered a consequentialist philosophical outlook because it both believes that outcomes can be predicted and because it judges actions based on their outcomes. Thus, utilitarianism is often associated with the phrase 'the ends justify the means.'
Deontology: Deontology is an alternative ethical system that is usually attributed to the philosophical tradition of Immanuel Kant. Whereas utilitarianism focuses on the outcomes, or ends, of actions, deontology demands that the actions, or means, themselves must be ethical. Deontologists argue that there are transcendent ethical norms and truths that are universally applicable to all people. Deontology holds that some actions are immoral regardless of their outcomes; these actions are wrong in and of themselves. Kant gives a categorical imperative to act...