Q. Is it mandatory to use a person’s full name the first time it is mentioned in the body of the text, and to use only the last name in subsequent mentions?
A. For most people with a first and a last name, it’s usually best to do what you suggest as a courtesy to your readers. But names come in so many varieties that we tend to avoid making absolute rules about them. For example, a passing mention of Shakespeare’s Hamlet wouldn’t need to include “William.” And there are those who have only one name, like Aristotle, or who are known by one name, like Elizabeth I (who is only sometimes referred to as Elizabeth Tudor, as in studies that highlight the Tudor dynasty and its lineage).
If you’re using author-date citation style, there’s another exception to consider: When citing an article or other source in parentheses, you’d normally include only the author’s last name even if that author hasn’t been mentioned elsewhere in the text: (Smith 2024). Interested readers can go to the corresponding entry in the reference list for the full form of the name. But if you’re referring to the author rather than the work (in what is sometimes called a narrative citation), it’s normally best to include the first name at first mention: “According to Ivy Smith (2024), . . . Smith, however, does not . . .”
For the difference between “Author (Year),” which refers to a person, and “(Author Year),” which refers to a work, see CMOS 13.122.