An official crowd of 94,199 fans poured into the iconic venue on Boxing Day, surpassing the previous record of 93,013 for the ODI World Cup final in 2015.
It also beat the previous record for a day of Test cricket, when 91,112 attended day one of the 2013 Ashes contest.
That match also drew the record total crowd for an Ashes Test - 271,865 - which MCC boss Stuart Fox hopes will be broken in coming days.
Huge attendances are also expected on days two and three - if the match lasts that long.
England are chasing their first victory in the five-Test series, while Australia are seeking to remain on track for a whitewash.
"It'll be a record today, but I'd love to shoot for one for the whole Test," Fox said before the first ball on Boxing Day.
"I think it was 2013 when we had 271,000 through the gate, so I think that's realistic.
"By the sign of things behind me, you can see the members' gates flying, so that's always a good sign."
Cricket Australia CEO Todd Greenberg said the number of travelling supporters from interstate and overseas for this summer's Ashes series had been "phenomenal".
"Nearly 35 per cent of people who come today to this Test match will be visiting the MCG for the very first time," Greenberg said.
"So it's incumbent on us, the sport, the venue, the government to give people a wonderful experience.
"When they're here today, they're going to watch some great cricket clearly, but it's more than about the cricket.
"It's actually bigger than cricket. It's bigger than about sport. This is one of the nation's great events."
Last year, Australia and India smashed the all-time attendance record for a Boxing Day Test when a total 373,691 fans saw the hosts' victory.
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