When Campbell returned to the United States he took up a professorship at Indiana University (1888 to 1891), writing the textbook Elements of Structural and Systematic Botany. In 1891 he became the founding head of the botany department at Stanford University and remained at Stanford for the remainder of his career, retiring in 1925. He studied mosses and liverworts, producing The Structure and Development of Mosses and Ferns in 1895. This book, together with its subsequent editions in 1905 and 1918, became the authoritative work on the subject and "firmly established Campbell's reputation as one of the leading botanists of the United States." His Lectures on the Evolution of Plants was published in 1899, and became widely used as a botany textbook. University Textbook of Botany was published in 1902, with fears expressed by colleagues that interest in pure research interest would prejudice its worth being found to be misplaced. He also travelled extensively though the Pacific collecting samples and writing Outline of Plant Geography, published in 1926, about his travels.
