Mary Cartwright proved many important theorems in the theory of entire functions (too many to list them here). For a survey of her contributions I recommend her obituary:
Zbl 1032.01034
Hayman, W. K.
Mary Lucy Cartwright (1900–1998),
Bull. London Math. Soc. 34 (2002), no. 1, 91–107.
written by Hayman, who also made many important contributions to entire functions.
Nowadays she is more famous for her contribution to non-linear dynamics (partially joint with Littlewood) but these papers were VERY much ahead of their time, and were not sufficiently appreciated until the late 1980s. Before that she was more famous for her contribution to the theory entire functions. For example, the class of entire functions of exponential type with
the property
$$\int_{-\infty}^\infty\frac{\log|f(x)|}{1+x^2}dx<\infty,$$
which plays a fundamental role in harmonic analysis, is called the Cartwright class. (The principal theorem about zero distribution of functions of this class is called Levinson's theorem, but it was proved independently by Levinson and Cartwright). Many of her results on entire functions were improved since then or absorbed into more general theories, but her book on entire functions remains one of her most cited works.