John Tyndall, Irish-born natural philosopher, completed his PhD at the University of Marburg in 1850 while starting his first substantial period of research into the phenomenon of diamagnetism. This paper provides a detailed analysis and evaluation of his contribution to the understanding of magnetism and of the impact of this work on establishing his own career and reputation; it was instrumental in his election as a Fellow of the Royal Society in 1852 and as Professor of Natural Philosophy at the Royal Institution in 1853. Tyndall's interactions and relationships with Michael Faraday, William Thomson, Julius Plücker and others are explored, alongside his contributions to experimental practice and to emerging theory. Tyndall's approach, challenging Faraday's developing field theory with a model of diamagnetic polarity and the effect of magnetic forces acting in couples, was based on his belief in the importance of underlying molecular structure, an idea which suffused his later work, for example in relation to the study of glaciers and to the interaction of substances with radiant heat.