Magistrate judges serve as judicial officers of the U.S. district courts and exercise the jurisdiction delegated to them by law and assigned by the district judges. Magistrate judges may be authorized to preside in almost every type of federal trial proceeding except for felony cases.

The position of magistrate was established by Congress in 1968 to replace the position of commissioner, a position that had served the federal judiciary since the 1790s. In the mid-1960s members of the congressional judiciary committees and judges on the Judicial Conference of the United States recognized the need for revisions to the commissioner system. Commissioners were still paid by a fee system, the appointment process varied from court to court, and there were no uniform criteria for selection. Members of Congress and federal judges also wanted to relieve the congestion of the federal court dockets by expanding the judicial responsibilities of the commissioners.

The Federal Magistrates Act of 1968 (82 Stat. 1107) created the new title and expanded the magistrates' authority to conduct misdemeanor trials with the consent of the defendants, to serve as special masters in civil actions, and to assist district judges in pre-trial and discovery proceedings as well as appeals for post-trial relief. The act also authorized a majority of district judges on any court to assign to magistrates "additional duties as are not inconsistent with the Constitution and laws of the United States."

Under the act of 1968, magistrates are appointed by the district judges of each district court and are required to be members of the bar of the highest court within the state where they serve. Full-time magistrates serve for a renewable term of eight years and part-time magistrates for a renewable term of four years. All magistrates are paid by salary. The Administrative Office of the U.S. Courts was made responsible for the administration of the magistrate system, and the act charged the Federal Judicial Center with providing training for magistrates. After implementation of a pilot program in five districts and the completion of surveys to determine the necessary number of magistrates, the new system was in place throughout the federal judiciary by July 1971.

The district courts' broad application of the grant of "additional duties" soon led to conflicting interpretations in the courts of appeals. In 1974 the Supreme Court ruled that magistrates could not conduct evidentiary hearings in habeas corpus actions. Congress responded in 1976 with an act (90 Stat. 2729) that further defined the magistrates' authority and granted them the power to conduct habeas proceedings. In the Federal Magistrates Act of 1979 (93 Stat. 643), Congress again expanded the authority of the magistrates by granting them so-called consent jurisdiction, which authorized them to conduct all civil trials as long as the parties consented. Magistrates were also allowed to preside over all misdemeanor trials as long as the defendants waived their right to a trial before a district judge. The act of 1979 also provided for merit selection panels to assist district judges in the appointment of magistrates.

The Judicial Improvements Act of 1990 (104 Stat. 5089) changed the office title from magistrate to magistrate judge. The number of magistrate judgeships is determined by the Judicial Conference of the United States, on the advice of district judges, the circuit councils, and the Administrative Office of the U.S. Courts, and is subject to congressional funding of the requested positions. As of March 2009 there are 517 full-time and 42 part-time authorized magistrate judgeships, as well as one position combining magistrate judge and clerk of court.