Born at Magheragall in Ireland, Larmor gained his BA and MA from Queen's University, Belfast; he entered Cambridge University in 1877, gaining a fellowship in 1880. He then became professor of natural philosophy at Queen's College, Galway. In 1885 he returned to Cambridge as a university lecturer in mathematics and in 1903 became Lucasian Professor of Mathematics. He retired from this post in 1932. Apart from his scientific work Larmor served as member of parliament for Cambridge University from 1911 to 1922.

Larmor's central interests were in applied mathematics and physics, specifically in electromagnetic theory, optics, mechanics, and the dynamics of the Earth. Like the work of his contemporary, Hendrik Lorentz, Larmor's work belongs to the final phase of classical physics that paved the way for the revolutions of relativity and quantum theory. An example of Larmor's basic scientific conservatism was his support of the concept of the ether as the wave-bearing medium thought to pervade all space and his work, published in 1900 as Aether and Matter, on the motion of matter through the ether. He believed that matter could only interact with the ether through the effects of electrically charged particles that formed part of the ether.

Larmor made two particularly important contributions to electrodynamics. He was the first to predict in 1897 the Larmor precession. This is the wobbling motion of the orbital plane of an electron moving in an atom when subjected to a magnetic field. The axis at right angles to the plane of the orbit sweeps out a conical area. Larmor also derived a nonrelativisitic formula that expresses the power radiated by an accelerated electron as being proportional to the square of the product of charge and acceleration.