Final answer:
Mason and Dixon arrived in Philadelphia before heading west. Philadelphia was a thriving port city with connections to the African slave trade and a magnet for immigrants from Europe.
Step-by-step explanation:
The Pennsylvania city that Mason and Dixon arrived in before heading west is Philadelphia.
Philadelphia was an important port city and a magnet for immigrants from all over Europe. The city also played a role in the African slave trade and had Quaker merchants who participated in efforts to abolish slavery. Philadelphia was the base for British operations during most of the Revolutionary War.