you need to make county_ID as index for the right frame:
frame_2.join ( frame_1.set_index( [ 'county_ID' ], verify_integrity=True ),
on=[ 'countyid' ], how='left' )
for your information, in pandas left join breaks when the right frame has non unique values on the joining column. see this bug.
so you need to verify integrity before joining by , verify_integrity=True