Niemeyer was born in Princeton, New Jersey. He attended Kenyon College, where he played on the school's baseball team. He then studied at the University of Munich, before pursuing his legal education at Notre Dame Law School. He was admitted to the Maryland bar and practiced commercial law at Piper & Marbury in Baltimore, Maryland, from 1966 to 1988. In 1984, Niemeyer co-authored the Maryland Rules Commentary, a treatise on the rules of procedure in the Maryland state courts. From 1973–88, he was a member of the Maryland Court of Appeals Standing Committee on Rules of Practice and Procedure. In 2006, Niemeyer published A Path Remembered: The Lives of Gerhart & Lucie Niemeyer. Niemeyer's father, Gerhart Niemeyer, was a political philosopher and professor of government at the University of Notre Dame. Niemeyer is married and has three sons. Niemeyer's father was a conservative political philosopher and friend of William F. Buckley, Jr. Upon Hitler's rise, in 1933, Niemeyer's father left Germany for Spain and then the US. Niemeyer, like his father, studied at the University of Munich. The New York Times obituary of June 29, 1997, states that Niemeyer's father: "wrote that fascism, communism and other such modern mass movements were the legacy of disoriented philosophers. He said their ideas corroded the cultural mettle of a society and spawned ideologies with a limited view of humanity."
On July 28, 2014, Niemeyer dissented from a 4th Circuit ruling that struck down Virginia's ban on same-sex marriage as unconstitutional. In his dissent, he argued that under a rational basis test Virginia's ban should be deemed constitutional. On April 19, 2016, Niemeyer dissented in part from a 4th Circuit ruling in an appeal from the US District Court for the Eastern District of Virginia at Newport News where the majority of the 4th Circuit panel reversed the district court's dismissal of a transgender boy's claims under Title IX. Niemeyer's dissent states: "This unprecedented holding overrules custom, culture, and the very demands inherent in human nature for privacy and safety... More particularly, it also misconstrues the clear language of Title IX and its regulations"; and "And finally, it reaches an unworkable and illogical result". The Majority rejected Niemeyer's assertions, concluding that "the record is devoid of any evidence tending to show that use of the boys' restroom creates a safety issue." Further, the Majority rejected Niemeyer's "suggestion that... the enforcement of separate restroom facilities impossible because it 'would require schools to assume gender identity based on appearances, social expectations, or explicit declarations of identity.' Accepting position would equally require the school to assume 'biological sex' based on 'appearances, social expectations, or explicit declarations of .' Certainly, no one is suggesting mandatory verification of the 'correct' genitalia before admittance to a restroom. The Department ’s vision of sex-segregated restrooms which takes account of gender identity presents no greater 'impossibility of enforcement' problem than does the 'biological gender' vision of sex-segregated restrooms." On May 25, 2017, Niemeyer wrote a dissent when the en banc circuit upheld a lower court's injunction against the President's travel ban by a vote of 10–3 in International Refugee Assistance Project v. Trump. The decision would later be overruled by the Supreme Court in Trump v. Hawaii. In March 2018, Niemeyer wrote a dissent when the circuit denied en banc rehearing to a divided panel's conclusion that the Bladensburg Peace Cross memorial from World War I now violated the Constitution's Establishment Clause. The Fourth Circuit's judgment was then reversed by the U.S. Supreme Court in American Legion v. American Humanist Association.
