This article traces the J. Walter Thompson Company's rise to prominence in the advertising industry against the backdrop of the growth of the American advertising industry, American big business, and the emergence of a mass consumer society. Furthermore, the article looks into the internal records of the J. Walter Thompson Company to highlight how the advertising agency itself defined "advertising" and explained some of its practices. It begins by providing a historical overview of the emergence of the modern advertising agency. In addition to examining the business history of the early advertising industry and how the J. Walter Thompson Company became the world's largest agency by the middle of the 20th century, the article also explores the economic and social roles of the advertising agency.