Johnson is an internationally recognized leader in surface science and photoelectron spectroscopy, a process in which an electron absorbs energy from light and is ejected from the material under study, thus revealing information about its properties. Johnson has focused on materials in which electrons are confined to two dimensions, rather than the usual three dimensions. Two-dimensional materials include the surfaces of metals, where catalytic reactions take place; magnetic multilayers, consisting of alternating layers of thin films of magnetic and nonmagnetic materials; and high-temperature superconductors.
After earning a Ph.D. in physics from Warwick University in 1978, Johnson worked for Bell Laboratories, and then joined Brookhaven Lab in 1982. He rose through the ranks to become a senior physicist in 2000, and he became Acting Associate Chair of Brookhaven's Physics Department in 2003. Johnson was appointed Deputy Chair of the Condensed Matter Physics & Materials Science Department upon its founding in April 2006. In June 2007, he became Acting Chair of the department. He also leads the department's electron spectroscopy research group.
A recipient of the American Physical Society's Oliver E. Buckley Prize in 2011 and Brookhaven Lab's Science and Technology Award in 2001, Johnson is a Fellow of the American Association for the Advancement of Science, the American Physical Society and the Institute of Physics in the United Kingdom. He is the author of 150 peer-reviewed scientific papers.