Demetriou announced his resignation last month after more than 10 years in charge, having expanded the AFL from 16 to 18 teams and strengthened the game's standing as the country's richest and most well-attended sports competition.
The 40-year-old McLachlan, an AFL staffer since 2000, was heavily backed to take over from Demetriou, whose legacy was tainted last year by a major anti-doping investigation which continues to overshadow the game.
"I understand the honour, I understand the responsibility, I understand the privilege of leading this code and this game," McLachlan said.
Tigers maul Crows to end long AFL title drought
While Demetriou's aggressive but calculated stewardship has put the game on a firm financial footing, expansion clubs in Sydney and the Gold Coast, traditional heartlands of rival code rugby league, remain vulnerable and are likely to be a drain on league coffers for years to come.
McLachlan may also face a more immediate test over the fate of the Melbourne-based Essendon Bombers, one of the AFL's oldest and most storied clubs, who could be hit with more sanctions over the administration of a dubious programme of supplements to players from 2011-12.
The Bombers were booted out of the play-offs last year and slapped with a record fine in the AFL's biggest scandal of the modern era, sparked by an investigation by Australia's national anti-doping agency.
No players have been punished, but a former judge is expected to submit his recommendations to the agency this week, local media have reported.
The AFL drew fierce criticism for their handling of the crisis, with Essendon's punishment ultimately haggled over by opposing lawyers in back rooms late last season.
McLachlan sought to draw a line under the episode.
"It was an incredibly tough period in the history of our game and we ended up in a position that I don't think was edifying for a lot of people and certainly wasn't great for the game," he said. "But we must look forward."
Demetriou, 53, was expected to hold the reins until the end of the current season, his 11th in charge, but will relinquish them on June 5, the AFL said.
Disappointing crowds in the opening rounds of the current season fuelled pundits' calls for him to step down quicker.
As chief operating officer, McLachlan played a key role in securing an Australian record A$1.25 billion broadcast rights deal for 2012-16, which has helped finance the league's expansion.
The distribution of revenues remains a sore point, however.
A longstanding Robin Hood-style equalisation policy in which poorer clubs receive a greater cut has not allayed fears that the 18-team league could end up dominated by a handful of rich clubs - in the same vein as some of Europe's top football leagues.
Describing his task as one of consolidating investments, McLachlan suggested further expansion was off the agenda. He struck a populist tone about helping fans weary of rising ticket prices and exorbitant food costs at stadiums.
"The most pressing (matter) I can think of is continued support and structural improvement in our weaker clubs, having strong competition," he said.
"I think also (it's) engaging with our fans and understanding the challenges and really, in my words, taking out any encumbrance whether financial or scheduling or logistical for them being able to pursue their passion and go to the football."
Aussie Rules coach Phil Walsh stabbed to death, son arrested
Jeering of Aboriginal player awkward for Australian sport
Share this article