The Socceroos all-time leader goalscorer struck five minutes before half-time, which doubled Shanghai's lead after Mohamed Sissoko's opener in the 2nd minute.
Cahill claimed his second goal in the 86th minute, giving his side an unassailable 3-0 lead in the process.
The 35 year-old has now scored an impressive 11 goals already this season in the Chinese Super League.