But the Gunners Ladies had to come from behind to claim victory, just as they did in the first meeting between the two sides earlier this season.
Natalie Preston gave United an 18th-minute lead with a nice chip following a through-ball by former England captain Karen Walker. But Sanderson equalised straight from a 36th-minute free-kick.
A minute before half-time a 15-yard effort by England winger Rachel Yankey gave the Londoners the lead.
The second half was equal until England striker Kelly Smith scored Arsenal's third goal from a 72nd-minute penalty.
Sanderson, who scored a late winner in Arsenal's 4-3 home win against Leeds in August, added her second with a close-range effort in injury time.