Milicic will see out the A-League season with Western Sydney Wanderers before joining the national team on a full-time basis ahead of this year's FIFA World Cup, starting in June.
Though, the Wanderers have allowed the highly-rated assistant to fly to London for Australia's international friendly against Ecuador on March 5.
Milicic is the third coach to progress from the A-League this season following Postecoglou's promotion to the Socceroos and Graham Arnold's departure to Vegalta Sendai in Japan.
"I'm honoured to have been handed such a tremendous opportunity to work as an assistant coach with the Socceroos," said Milicic, who scored five goals in six appearances for the Socceroos between 2002 and 2005.
"As a former Socceroo I am passionate about the National team and I look forward to the challenge.
"I'm excited to be able to work with Ange, especially with the World Cup coming up as well as the Asian Cup in Australia and with so many talented players starting to break through."
Milicic has become one of Australia's most sought-after coaches following stints with Sydney United, Melbourne Heart, the Young Socceroos and Western Sydney.
The 39-year-old was unfortunate to lose out to John Aloisi for Heart's head coaching role before joining forces with Tony Popovic in Western Sydney, where the pair guided the Wanderers to the premiership and a grand final appearance in the club's first season.
"I've had a good apprenticeship under John van't Schip and Poppa (Tony Popovic) as well as the 2009 and 2011 U20 World Cups as an assistant to Jan Versleijen and I'm looking forward to continuing that under Ange.
"I'd also like to thank the Western Sydney Wanderers for allowing me to take this opportunity and I am determined to finish the A-League season with the same passion and dedication I will take to the National Team."
Aurelio Vidmar will also remain as Postecoglou's assistant and continue with his duties as head coach of the Olyroos.
"I'm delighted that Ante Milicic will be coming on board as an assistant coach with the Socceroos," Postecoglou said.
"One of my priorities after becoming National Team Head Coach was to secure quality staff to help drive the Socceroos and Ante is a driven coach who will fit into our environment.
"I wanted the best people to come in and contribute to the growth and success of the National Team and I'm confident Ante will add further expertise to our existing staff.
"Ante has experience at domestic (Hyundai A-League) level and International level with both senior and junior National teams and will play a part in an exciting new era as we look to build the next golden generation of Australian footballers.
"I'd like to take this opportunity to thank Western Sydney Wanderers for their cooperation to ensure I was able to appoint the people I thought were best for the job."
