The Melbourne Victory coach received 36.8 per cent of a fan poll of 15,000 votes.
He shaded Western Sydney Wanderers coach Tony Popovic, who garnered 36.2 per cent of the votes.
Fans will also be involved in choosing the All Stars squad.
They will be responsible for 50 per cent of the voting, with 20 per cent going to an expert panel, another 20 per cent via the PFA Team of the Year, with the remaining 10 per cent going to Postecoglou.
He will also have 100 per cent jurisdiction over an additional three players in the 18-man squad.
Postecoglou could be without up to 15 A-League players, because the Socceroos will be involved in the East Asian Cup finals the same week.
However the Victory mentor was confident the A-League had the depth and emerging young talent to be very competitive against United, who look certain to win this year's English Premier League title.