The England international, 30, was playing her last game for the Gunners before returning to life as a professional player with Boston Breakers in the new US-based Women's Premier League.
Alex Scott, also joining the Breakers, and Suzanne Grant also scored in the 5-0 victory over Doncaster Rovers Belles.
'I'm going to miss everyone involved with the club. It's been a fantastic four years and I'm sad to be leaving, but excited about the prospect of playing professionally in America," said Smith.