In 1946, Pais served as a personal assistant to Niels Bohr at the Bohr's country home in Tisvilde. In 1956, he became a naturalized citizen of the United States. His autobiography A Tale of Two Continents (Princeton, 1997) refers to the 'esemplastic power of the imagination'. Pais was perhaps best known for his biography of Albert Einstein, Subtle is the Lord (Oxford, 1982), and sequel, Einstein Lived Here. He also wrote a history of physics in the 20th century, Inward Bound (Oxford, 1986), a biography of Niels Bohr, Niels Bohr's Times, In Physics, Philosophy and Polity (Oxford, 1991), and The Genius of Science: A Portrait Gallery (Oxford, 2000).