William Isaac Thomas working with Polish sociologist Florian Znaniecki, developed innovative work on the sociology of migration. Through his Thomas theorem, Thomas contended that, "If men define situations as real, they are real in their consequences".
William Isaac Thomas also taught many courses in Latin, French, German and Greek. Thomas introduced the a concept of the 'definition of the situation', which was later referred to as the Thomas theorem.
With the ideas of George Herbert Mead, his concept of the defining the situation proved to be an important part of the rebellion of symbolic interactionism against structural functionalism. In 1927, thanks to the support of a younger generation of scholars and against opposition from the establishment, Thomas was made honorary president of the American Sociological Society.
According to Thomas' "definition of the situation", prior to making a decision, people "generally examine and deliberate about occurrences before acting". - Sands, Roberta G. 2014. “William I. Thomas, The unadjusted girl”. Qualitative Social Work, 13:725-728.
1907 saw the publication of Thomas's first major work, Sex and Society. In "Sex and Society", Thomas speculated that women's intellect might actually be superior to men's "due to their superior cunning" and "superior endurance".
Thomas writes "The sociopsychological aspects of culture history, or otherwise stated, social psychology as examined in relation to races, nationalities, classes, interest groups, etc., in different cultural situations and historical epochs; and second, personality development in normal, criminal and psychopathic individuals in relation to cultural situations and particular trains of experience as seen through their life-histories, which may be in the form of autobiographies, case studies, continuous and organized inter-views, etc. (I do not say `psychoanalysis' because of the meaning which this term has acquired)." - "William Isaac Thomas". American Sociological Association.
According to an anecdote told by Thomas himself, it was an accident that inspired him to use personal written material as primary ethnographic sources and to develop the biographical approach to sociology that would make his lasting reputation in the field. According to Thomas' anecdote, while he was walking down a street near his home, Thomas was nearly hit by a garbage bag that had been thrown out of a window. The bag burst open on the sidewalk. Then, Thomas discovered a letter in it that had been written by a Polish immigrant.