You either love Mill or hate him. Marx, a near contemporary of Mill, was in the latter group, for a chasm of thought separated the two men. Marx, the collectivist who saw personal liberty as an impediment to revolution, differed from the individualist Mill, who regarded it as the essence of dignified living. Yet Mill would have sat uneasily in any political party today. He was a traditional liberal devoted to freedom, who sometimes called himself a socialist, though Socialism in Mill's time was a concept wider than it is now, but he ended his career in the Conservative party. A devoted supporter of women's equality, he was an early advocate of votes for women. In some ways my support for Mill is strange, as I am a religious believer, while he was an atheist, but we can find good in those with whom we disagree, and there was much good in J.S.Mill. I, like Mill, am with all those being persecuted for their beliefs or witch-hunted for lifestyles that do not conform to custom, majority practice or the values of a dominant class. In the Middle Ages I would not have been hunting witches, but hiding them.
The key to Mill's political thought is the harm principle, which is the foundation of his political ethics. The principle is as follows; the only justification for interfering with the liberty of another person is to prevent harm to others. This is a profoundly egalitarian statement, because it insists that people can speak as they wish and live as they wish, as long as they do not harm another person. He limited this right to adults, for he realized that children had to be subjected to restraint, and he was aware that there are adults who need controlling for their own good, but in general Mill believed that in normal circumstances most adults should be free to live and act as they wished.
Now, Mill was aware of the excuses made by illiberal people, who want to interfere with others' liberty. The first is the common modern excuse, offence. I find your views offensive so I complain to the police. Mill would have no truck with this, as he believed that offence was not harm. That I dislike a person's views is not evidence that I am harmed in any way. Nor would Mill accept arguments that a view might cause harm and therefore should be banned, an argument beloved of authoritarians and persecutors throughout history. He would reply that the harm has to be demonstrated and measurable, not merely possible and distant.
It would be wrong to think that Mill believed there should be no constraints at all on free speech. He gives the example of someone shouting "fire" in a crowded theatre, when there is none, which causes fear and threat to life from stampede, when no such threat is present. He also says that calling corn dealers criminals when an angry mob is at their doors is not acceptable, as situations might get out of control. He was not arguing for incitement, and would have no truck with bullying of any kind, but he believed that the expression of arguments and views should be unimpeded by law or social pressure.