mˈænsfiːld wʌz bˈɔːɹn ɪn ɹˈʌksli, kˈɛnt, ðə fˈɪfθ ʌvðə sˈɛvən sˈʌnz ʌv dʒˈɑːn mˈænsfiːld ʌv dˈɪɡɪswˌɛl hˈaʊs ɪn hˈɜːtfɚdʃɚ, ænd hɪz wˈaɪf, mˈɛɹi bjuːkˈænən smˈɪθ, dˈɔːɾɚɹ ʌv dʒˈɛnɚɹəl sˈæmjuːl smˈɪθ ʌv bˈɔltɪmˌoːɹ ɪnðə juːnˈaɪɾᵻd stˈeɪts.
Mansfield was born in Ruxley, Kent, the fifth of the seven sons of John Mansfield of Diggeswell House in Hertfordshire, and his wife, Mary Buchanan Smith, daughter of General Samuel Smith of Baltimore in the United States.