The major difference here is that Mill's utilitarianism is a consequentialist theory of morals while Kant's is emphatically not. Mill says that the morality of an action is determined by looking at the impacts that it has on people. By contrast, Kant says that the morality of an action is determined by the intention behind it.
To Mill, an action is moral if it brings a net gain in happiness to people. The intention behind the action is irrelevant. Kant's categorical imperative says that the actor must always act in such a way that the rule behind the action could be made into a universal law.
To Mill, an action could be moral even if the person taking it was trying to do something bad. To Kant, an action could be moral even if it ended up creating bad effects. This is the major difference between these two views.