Parcels v. Johnson was a United States Supreme Court case that dealt with the issue of whether a state could tax a federal government employee's salary. The case was brought by a federal employee, Parcels, who argued that the state of Maryland had no right to tax his salary, as it was paid by the federal government. The Supreme Court ultimately ruled in favor of Parcels, holding that the state of Maryland could not tax a federal employee's salary. The Court reasoned that the federal government had exclusive authority over its own employees, and that the state of Maryland could not interfere with this authority by taxing the salary of a federal employee. The Court further held that the state of Maryland could not tax the salary of a federal employee because it would be a violation of the Supremacy Clause of the United States Constitution. The Court's decision in Parcels v. Johnson was an important one, as it established that the federal government had exclusive authority over its own employees, and that the states could not interfere with this authority by taxing the salary of a federal employee. This decision has been cited in numerous cases since, and has been used to protect the rights of federal employees from state taxation.

Justice Field delivered the dissenting opinion in Parcels v. Johnson, arguing that the majority's decision was inconsistent with prior Supreme Court precedent and would lead to a dangerous expansion of federal power. He argued that Congress had no authority under the Constitution to pass legislation granting exclusive jurisdiction over cases involving title or boundaries of land to state courts, as it did in this case. The only way for such an act to be constitutional is if it falls within one of Congress' enumerated powers, which he argued does not include this type of legislation. Furthermore, Justice Field noted that there were numerous other instances where similar acts had been passed by Congress without any challenge from either party involved in those cases; thus indicating their acceptance and approval by both parties at the time they occurred. Finally, he concluded his dissent by noting that if allowed to stand unchecked then future generations could suffer greatly due its potential implications on civil rights issues related to property ownership and boundary disputes between states or individuals residing within them.