The Australian initially joined Millwall as a 17-year-old in 1997, and spent seven years with the London club - scoring 57 goals in 251 appearances - before leaving to join Everton in 2004.
Cahill has since represented the likes of New York Red Bulls and Melbourne City during a well-travelled career, but it was announced on Monday that the now 38-year-old had returned to Millwall on a contract until the end of the season.
Cahill has vowed to help "on and off the park" during his second spell with the Lions.
"It feels amazing [to return]. This was a big decision in my footballing career," Cahill told reporters. "To come home where it all started is very special. From the opportunity the club gave me as a kid to now coming back and try and make an impact for the team - I just want to enjoy it.
"I have had a couple of sessions with the team and it just feels like I have never left. The lads have been brilliant, the whole setup is the same and I am very thankful for the opportunity.
"I know what the badge is all about, I know what the fans want and that's players to be selfless, and to be part of the group. So whatever I can do to help on and off the park I will do that."
Cahill could make his second Millwall debut against Reading on Saturday afternoon.