Ex parte Lothrop was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The petitioner, Lothrop, was a prisoner in the custody of the United States Marshal in the District of Columbia. He had been convicted of a crime in the District of Columbia and was serving his sentence in the federal prison. Lothrop sought a writ of habeas corpus from the state court in the District of Columbia, claiming that his conviction was unconstitutional. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The Court reasoned that the writ of habeas corpus was a federal remedy and that the state court did not have the power to interfere with the federal government's authority to imprison a person convicted of a federal crime. The Court also noted that the writ of habeas corpus was a remedy available only to those who were in custody in violation of the Constitution or laws of the United States. Since Lothrop was in federal custody, the state court did not have the authority to issue the writ.

In Ex Parte Lothrop, the Supreme Court was tasked with determining whether a writ of habeas corpus should be issued to release a prisoner who had been held in custody for over two years without being brought before a court. The majority opinion found that the writ should not be issued because there was no evidence that the petitioner's imprisonment violated any constitutional rights or laws. However, Justice Field dissented from this decision and argued that it is unconstitutional to hold someone in prison indefinitely without bringing them before a court. He further argued that even if there were some legal justification for holding an individual in prison, such as suspicion of criminal activity or mental illness, they must still be given their right to due process under law by appearing before a judge within reasonable time limits. Justice Field concluded his dissent by stating "the great object of all governments is protection of life and liberty; but when these are taken away arbitrarily and without cause shown...it becomes oppressive."