The policies that Mill advocates might be called utopian because they posit ideal conditions in which people make the most reasonable decisions without taking account of practical politics or self-interest.
John Stuart Mill is the ultimate classical liberal. The three policies for which he is best known are utilitarianism, freedom of speech, and the liberty of the individual. He also held progressive views on women's rights, racial equality, and the abolition of slavery, among many other matters.
Mill's utopianism, however, is perhaps most clearly seen not in what he thought about matters of policy but in how he thought about them. Though he was a member of Parliament who also participated in government as a civil servant, he shows little interest in practical politics in such works as Utilitarianism and On Liberty. Even in Principles of Political Economy, he takes it as read that people will adopt the most logical system overall rather than be motivated by self-interest, as Marx and other political thinkers believed they would.