Mulhall v. Keenan et al. was a United States Supreme Court case that addressed the issue of whether a labor union had the right to require a non-member to pay a fee in exchange for the union's services. The plaintiff, Mulhall, was a non-member of the union and was required to pay a fee in order to receive the union's services. Mulhall argued that the fee was a violation of his rights under the Fourteenth Amendment. The Supreme Court held that the union's fee was not a violation of the Fourteenth Amendment. The Court reasoned that the union was providing a service to Mulhall, and that the fee was a reasonable compensation for that service. The Court also noted that the fee was not a penalty or punishment, and that it was not imposed as a condition of employment. The Court's decision in Mulhall v. Keenan et al. established that labor unions have the right to require non-members to pay a fee in exchange for the union's services. This decision has been cited in numerous subsequent cases involving labor unions and their rights.

Justice Field delivered the dissenting opinion in Mulhall v. Keenan et al., arguing that Congress had no authority to pass a law granting exclusive rights of fishing in navigable waters to any individual or company. He argued that such an act was unconstitutional because it would be taking private property for public use without just compensation, which is prohibited by the Fifth Amendment. Furthermore, he argued that this type of legislation could not be justified under the Commerce Clause since it did not involve interstate commerce and thus should have been left up to state legislatures instead of being regulated by federal law. Justice Field concluded his dissent with a reminder that if Congress were allowed to grant exclusive privileges like these, then they could potentially do so for other industries as well and thereby create monopolies which would ultimately harm consumers and stifle competition in violation of basic principles underlying our free market system.