Concern in the United States for adequate public school education in mathematics dates back at least to the response to Sputnik in 1957, but at that time focused on preparing a cadre of students for quantitatively demanding careers in science and engineering. More recently, however, there has been increasing appreciation of the value of broadly distributed aptitude in application of elementary mathematical reasoning to quotidian concerns. This idea of numeracy, or quantitative literacy, dates back at least to a British government report of the late 1950s, but the concept was popularized by John Allen Paulos in his book "Innumeracy." While the exact definition of quantitative literacy varies according to context, assume here that it means the ability and the habitual tendency to engage quantitative information in an effort to understand the world. Quantitative literacy is often justified as a critical skill for economic success in a knowledge-based, globally-competitive economy. Less frequently it is acknowledged to be a key ingredient in the formation of an informed and engaged citizen in contemporary democratic society. This piece describes a first-year college seminar course that demonstrates the utility of quantitative literacy in identifying, understanding, and addressing issues of social justice. Social justice is another concept that is defined in different ways depending on context, but it is not widely seen as having a substantial intersection with mathematics. The definition offered here is broad enough to encompass most commonly acknowledged issues of social justice, and politically neutral enough to be acceptable to most readers. It is based on Aristotle's theory of distributive justice discussed in the "Nicomachean Ethics": Social justice concerns the fairness of distribution of the benefits and burdens that arise from living in a community. Because fair distribution is fundamentally a quantitative idea, mathematics is a critical ingredient (though certainly not the sole ingredient) for a healthy appreciation of social justice. This essay provides a context for the course, a description of it, and the motivation for offering it, and demonstrates how its design suits that motivation. It includes responses of various stakeholders to the course, and reflects on learning opportunities the course provides.