McHenry was born Mary Elizabeth Williamson Murphy in Washington, D.C., to  Alphonso Williamson (who worked with the Library of Congress) and Elizabeth Bennett Williamson (a teacher).
mək hˈɛnɹi wʌz bˈɔːɹn mˈɛɹi ᵻlˈɪzəbəθ wˈɪljəmsən mˈɜːfi ɪn wˈɑːʃɪŋtən, dˈiː.sˈiː., tʊ ælphˈɑːnsoʊ wˈɪljəmsən hˌuː wˈɜːkt wɪððə lˈaɪbɹɛɹi ʌv kˈɑːŋɡɹəs ænd ᵻlˈɪzəbəθ bˈɛnɪt wˈɪljəmsən ɐ tˈiːtʃɚ.