Mill's libertarian philosophy as stated in his essay On Liberty and the issues of wearing a mask in these times.

John Stuart Mill and the origins of the Libertarian philosophy
Today in this time of Covid the question of wearing masks has arisen.  For many the response is ‘You can’t make me wear a mask.  That infringes my liberty!’

Where does this idea of the individual as sovereign over her own self come from?  What is the source of this basic Libertarian philosophical principle?
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One of the major sources is John Stuart Mill’s book On Liberty, published in 1859.  Here are some key quotes from that book and the key doctrine it lays out.
(From Wikipedia: https://en.wikipedia.org/wiki/Harm_principle)
The belief "that no one should be forcibly prevented from acting in any way he chooses provided his acts are not invasive of the free acts of others" has become one of the basic principles of libertarian politics.[2]
 
The harm principle was first fully articulated by the English philosopher John Stuart Mill (1806–1873) in the first chapter of On Liberty (1859),[1] where he argued that:
 
“The object of this Essay is to assert one very simple principle, as entitled to govern absolutely the dealings of society with the individual in the way of compulsion and control, whether the means used be physical force in the form of legal penalties, or the moral coercion of public opinion. That principle is, that the sole end for which mankind are warranted, individually or collectively, in interfering with the liberty of action of any of their number, is self-protection. That the only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others. His own good, either physical or moral, is not a sufficient warrant. He cannot rightfully be compelled to do or forbear because it will be better for him to do so, because it will make him happier, because, in the opinion of others, to do so would be wise, or even right... The only part of the conduct of anyone, for which he is amenable to society, is that which concerns others. In the part which merely concerns himself, his independence is, of right, absolute. Over himself, over his own body and mind, the individual is sovereign. — John Stuart Mill[3]
Even if a self-regarding action results in harm to oneself, it is still beyond the sphere of justifiable state coercion.  Harm itself is not a non-moral concept. The infliction of harm upon another person is what makes an action wrong.[4]
 
Harm can also result from a failure to meet an obligation. Morality generates obligations. Duty may be exacted from a person in the same way as a debt, and it is part of the notion of duty that a person may be rightfully compelled to fulfill it.[3][4]
In the same essay, Mill further explains the principle as a function of two maxims:
The maxims are, first, that the individual is not accountable to society for his actions, in so far as these concern the interests of no person but himself. Advice, instruction, persuasion, and avoidance by other people, if thought necessary by them for their own good, are the only measures by which society can justifiably express its dislike or disapprobation of his conduct. Secondly, that for such actions as are prejudicial to the interests of others, the individual is accountable, and may be subjected either to social or to legal punishments, if society is of opinion that the one or the other is requisite for its protection. (From On Liberty Chapter V, 2nd paragraph)
 
The second of these maxims has become known as the social authority principle.[5]
 
However, the second maxim also opens the question of broader definitions of harm, up to and including harm to the society. The concept of harm is not limited to harm to another individual but can be harm to individuals plurally, without specific definition of those individuals.
 
This is an important principle for the purpose of determining harm that only manifests gradually over time—such that the resulting harm can be anticipated, but does not yet exist at the time that the action causing harm was taken. It also applies to other issues—which range from the right of an entity to discharge broadly polluting waste on private property, to broad questions of licensing, and to the right of sedition.
Note Sources:
2) Hamowy, Ronald, ed. (2008). The Encyclopaedia of Libertarianism. Thousand Oaks, California: Sage Publications. p. xxi. ISBN 978-1412965804.
3) Mill, John Stuart (1859). On Liberty. Oxford, England: Oxford University. pp. 21–22. Retrieved February 27, 2008.
4) Menezes Oliveira, Jorge (2012). "Harm and Offence in Mill's Conception of Liberty". Oxford, England: University of Oxford. p. 13.
5) Rossi, Philip J. (2012). The Social Authority of Reason. Albany, New York: SUNY Press. p. 60. ISBN 978-0791483367.

To sum up Mill’s philosophy of Libertarianism succinctly : One’ individual liberty to act ends when it impinges on someone else’s liberty to act to their detriment.  Or even more simply: I must not act if my actions harm another.  In the case of such possible harm a society has the right to intervene according to Mill.
Now at the time Mill was writing the general principles of physics and thus of actions and reactions was basically modeled on the Newtonian mechanistic model.  The Newtonian model is easily conceived of as actions and reactions between solid objects such as billiard balls.

However that model is not sophisticated enough to describe the interactions of human beings in the context of reality.  For example, it is seemingly okay for a person to smoke cigarettes under the Mill principle and not to expect any societal or governmental interference in the liberty of the individual to so choose to smoke.  However, that doesn’t account for the verifiable and document harm that is created by the phenomenon of second hand smoke.  Which Mill at his time and under the simple Newtonian physics model of biology would not have dreamed of.  Hence, the act does come under Mill’s idea of the liberty of others is being harmed by an act of the individual and thus can be curtailed.
Mill’s initial laying out of Libertarianism thus can deal with a world more complex than the simple view of the world as conceived of in 1880’s.

Now, let us consider the issue of the individual claiming his right not to wear a mask in these times within the context of Covid.
It has been verified that Covid is transmissible via contaminated water droplets expelled by a person’s mouth or nostrils.  It has also been verified that such transmission can be contained and prevented by having a person wear a mask.

Therefor the failure to wear a mask impinges on and threatens the liberty and life of other people.  Thus, according to Mill’s Libertarian philosophy society and the government has the right and the duty to enforce people to wear masks to prevent harm to life and liberty of the societies citizens.

Thus, there is no libertarian doctrine that permits a person not to wear a mask.
Failure to wear a mask is not only selfish but it is dangerous.