Complex sentence: He spent more than a year as chief of the fundamental research branch of the United States Atomic Energy Commission before becoming executive director of the American Mathematical Society in November 1949 where he served until 1954.
Split into simpler sentences: He spent more than a year as chief of the fundamental research branch of the United States Atomic Energy Commission. In November 1949 he became executive director of the American Mathematical Society where he served until 1954.