In front of 84,205 fans at the MCG - the largest crowd since the 2019 grand final because of the global pandemic - small forward Ginnivan starred in his side's 15.3 (93) to 12.10 (82) victory.
The 19-year-old had just 12 disposals but was awarded the Anzac Day Medal as best afield for his match-winning haul in the traditional blockbuster.
The result squared the Magpies' season record at 3-3 and left Essendon reeling at 1-5, with their finals hopes just about shot.
There were some nervous moments for Collingwood as the lead changed hands six times in the second half before they kicked three goals in four minutes at the start of the final quarter.
It gave them breathing space but the result wasn't settled until deep into time-on, when a 50-metre penalty against Nick Hind for moving on the mark put Brodie Grundy within range to kick the sealer.
Jordan De Goey (27 disposals) starred in the midfield and Jeremy Howe stood firm in defence as Brody Mihocek booted four goals.
Scott Pendlebury's (25 disposals) experience was important in his 15th appearance on Anzac Day and Nick Daicos (22) relished the big stage.
Essendon ball magnet Darcy Parish racked up a career-high 44 disposals and 12 clearances, while ruckman Sam Draper worked tirelessly and Alec Waterman (four goals) and Jake Stringer (three) had an impact in attack.
The Bombers got returns on their gamble of bringing back Stringer and on-baller Zach Merrett (36 disposals) back from injuries earlier than first expected.
Both sides entered the round-six contest desperate for a win to boost their finals hopes.
Collingwood lost Nathan Kreuger to a recurrence of his left shoulder injury during the second term but were invigorated by Jack Crisp's running goal and a bomb from Isaac Quaynor, which gave them a game-high 17-point lead before halftime.
Parish had 30 disposals and six clearances in the first half alone, helping Essendon to a 22-13 clearance advantage, but they lacked pace and penetration in possession.
And for all Parish's touches, De Goey had been even more influential to that point with his 16 disposals and three clearances for Collingwood.
Jayden Laverde was substituted out with a leg injury but the Bombers emerged from the main break with greater energy.
They hit the front for the first time at the 18-minute mark of the third term when Matt Guelfi pinned De Goey holding-the-ball and nailed the long set shot.
It was the first of four lead changes for the quarter as Ginnivan's fourth major gave the Pies a one-point lead at the final change.
The goal-for-goal arm wrestle continued early in the last quarter until quick goals to Mihocek, Ginnivan and Reef McInnes gave Collingwood a buffer.
Essendon recovered from dire positions at 1-4 and 2-6 last year to reach the finals but will need to engineer a special turnaround to repeat the feat this season.
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