The Spanish midfielder marked his debut in Australian football off the bench with a 72nd-minute free-kick that took a big deflection and trickled into the hosts' goal.
With this win, Macarthur became just the sixth side in the competition's history to secure a victory in its debut A-League match.
It topped an impressive cameo for Benat, introduced only four minutes earlier, who completed each of his 15 passes – 12 of which were in the opposition half – while both of his two shots were on target at Bankwest Stadium.
It may have been a different story had a VAR check not led to referee Kurt Ams overruling an early penalty against Aleksandar Jovanovic, having initially judged he had handballed Graham Dorrans' long-range strike.
But Macarthur had other chances of their own with Moudi Najjar, who had three shots and a key pass, heading over a cross from dangerman Ivan Franjic (three key passes, 86.3 per cent pass accuracy and four good crosses).
Adam Federici impressed in goal for the Bulls, saving a stinging effort from Simon Cox just before half-time. In total the ex-Reading keeper made six saves for this team, helping to consign Carl Robinson to defeat in his first game as Wanderers coach.