If the clerk entered her name as Mighill for her birth record, then that should be her LNAB, with McGill in the Other Last Name(s) slot.
I'm assuming here that neither the birth record nor the marriage record have suffered from having spellings "regularized."
Blodgette & Jewett, ''Early Settlers of Rowley, Massachusetts,'' rev. ed., 1953, pp. 234 ff., has lots and lots of Mighills in Rowley. (Sarah's father, Stephen is no. 127-8, p. 237. Sarah is no. 127-22, p. 237.) If you need a copy of this entry, let me know.