This chapter presents aspects of the work of Charles Sanders Peirce showing that he adhered to a number of pre-structuralist themes. Further, it indicates that Peirce’s position is similar in spirit to the category theoretical structuralist view of Steve Awodey (2004). The first part documents Peirce’s extensive knowledge of, and contribution to, the mathematics of his time, illustrating that relations played a fundamental role. The second part addresses Peirce’s characterization of mathematical reasoning as diagrammatic reasoning, that is, as reasoning done by constructing and observing rational relations in diagrams.