New Arsenal signing Lacazette who moved to the North London club last week in a £52.5 million deal from Lyon, came off the bench in the second half to score Arsenal's second goal before more than 80,000 fans at Sydney's Olympic stadium.
Lacazette side-footed home to beat goalkeeper Andrew Redmayne from close range after skipper Per Mertesacker had opened the scoring for the Gunners in the fourth minute.
Up to Lacazette's slick finish Redmayne had denied the Gunners with a string of spectacular saves to give Sydney FC an outside chance of snatching a draw against the English aristocrats.
The Gunners got off to a dream start when defender Mertesacker latched onto an overhead kick off a corner from fellow German Mesut Ozil to put his team ahead in the opening minutes.
Mertesacker pivoted and let fly to beat Redmayne from close range.
Youngster Reiss Nelson was lively down the right wing and troubled the home defence before his lack of fitness caught up with him late in the half.
Arsenal received a dubious penalty decision 10 minutes into the second half when Theo Walcott's shot hit scrambling Sebastian Ryall's back but referee Shaun Evans signalled hand ball.
But Redmayne again came to Sydney's aid with a diving save to his right to keep out Danny Welbeck's spot kick.
Halt time changes
Manager Arsene Wenger made mass changes midway through the second half, taking off all 10 of his outfield players and among his substitutes on came the club's new record signing Lacazette.
Redmayne again produced another acrobatic save to keep out an Alex Oxlade-Chamberlain free kick.
But Lacazette delivered the clinching goal when he finished off Alex Iwobi's cross from the right to find the net.
Arsenal play the second of a double-header of friendly matches in Sydney against Western Sydney Wanderers on Saturday before they head to China.