The Socceroos all-time leading goalscorer is believed to be returning to New York to live with his family, after a largely unsuccessful spell at his first professional football club.

It's unclear whether he'll retire completely after the 2018 World Cup. 

In 10 appearances, with no starts, Cahill failed to find the back of the net but was praised immensely by teammates and coach Neil Harris, as the 38-year-old's arrival coincided with the club's surge in form, with Millwall currently eighth in the Championship.

Cahill played a total of 63 minutes for the South-London club, 27 less than he had played in six matches for Melbourne City, before leaving the A-League club in March. 

His second spell ended in incongruous fashion as the Socceroo was banned for three matches towards the end of the Lions' unsuccessful promotion push. In a statement on Twitter, Cahill thanked the club for the opportunity.

"Time flies when your (sic) having fun. Massive thank you to everyone in the club, the fans and most importantly the players," Cahill wrote.

"A lot to be proud of this season and I have had the best time being back home where my career began. Blessed to have had this opportunity."