What is Mill's response to the claim that utilitarianism is a doctrine of expediency?

In Chapter 2 of Utilitarianism, Mill says that some people will accuse utilitarianism of being a philosophy that is really just about expediency.  Therefore, they will say, it has nothing to do with what is right.  What this means is that people think that utilitarianism says that you can just do whatever is easiest or whatever is best for you at a given moment.  Mill, however, rejects this criticism.

According to Mill, this criticism of utilitarianism is based on a poor understanding of what is expedient.  Mill says that things that people say are expedient really are not.  For example, he says that a person might really want to tell a lie to get themselves out of a tight spot.  However, he says, this is not really what is best for that person.  He says that a person who tells lies is essentially ruining his or her reputation.  In the long run, this is going to be very bad for the person.  No one will trust that person and that will be much worse than the predicament that the person lied to get out of.  As Mill says,

…it would often be expedient, for the purpose of getting over some momentary embarrassment, or attaining some object immediately useful to ourselves or others, to tell a lie.

However, as he goes on to say,

…the cultivation in ourselves of a sensitive feeling on the subject of veracity, is one of the most useful, and the enfeeblement of that feeling one of the most hurtful, things to which our conduct can be instrumental…

What he is saying here is that telling the truth is one of the most useful things to us, and a lack of honesty is one of the things that is most hurtful to us. 

So, what Mill is saying is that utilitarianism does not approve of actions that people would typically call expedient.  These actions really only seem to help people but, in the long term, they are harmful.  Because they are harmful, utilitarians would not approve of them.  Utilitarianism, then, is not a doctrine that approves of things that are merely done from what the world calls expediency.

In Utilitarianism, what is the objection that utilitarianism is a doctrine of expediency and Mill's response?

In the second chapter of this essay, entitled "What Utiltarianism Is," Mill identifies and responds to a number of criticisms that have been raised against utilitarianism as a philosophical position. One of these is the way in which utilitarianism has been identified as being the same as expediency, and is therefore considered to be immoral. Mill counters this attack on utilitarianism by arguing that expediency is being misinterpreted. He examines it and chooses to define as as acting in a way that is opposite to what is "right" in order to meet short-term goals and for personal interest. Mill takes the example of somebody choosing to tell a lie in order to avoid short-term embarrassment:

When it means anything better than this, it means that which is expedient for some immediate object, some temporary purpose, but which violates a rule whose observance is expedient in a much higher degree. The Expedient, in this sense, instead of being the same thing with the useful, is a branch of the hurtful.

Expediency therefore is actually defined as being negative because of the harm that it causes. Mill thus argues that expediency cannot be viewed as being useful, as harming society cannot be considered to be expedient. To act against the interests of society at large is to be immoral. Therefore Mill argues that expediency cannot be conflated in any way with utilitarianism, as it is expediency that is immoral, and not utilitarianism. He argues that the two terms are very separate and different, and cannot be confused with one another.