However, Easter struck in the dying embers, as the visitors slip two points behind Burnley in the race for automatic promotion.
QPR boss Harry Redknapp, who celebrated 30 years in management this week, watched on as Kranjcar came within millimetres of toasting to his boss's milestone in style.
The Croatia international allowed a dropping ball to bounce over his shoulder, before unleashing a 25-yard volley which crashed off goalkeeper David Forde's crossbar.
The 29-year-old would not dwell on the disappointment for long, though, playing a neat one-two with Joey Barton before smashing past Forde from 20 yards to open the scoring.
The Lions responded well to the deficit and should have provided an immediate rispote, only for Liam Trotter's close-range header to cannon into the side-netting.
Such were the poverty of Millwall's labours throughout the opening half, QPR extended their clean-sheet record to 800 minutes without conceding a goal.
McDonald, playing his first game in a month following a knee injury, brought this 43-year record to an end within seven minutes of the second half, though, converting Trotter's cut-back past goalkeeper Robert Green.
However, the hosts would not be level for long. Kranjcar turned provider to set up marksman Austin, who stroked past Forde to re-establish his team's lead.
Junior Hoilett should have put the result beyond doubt soon after, only to be denied by the Republic of Ireland shot-stopper.
It would prove a costly miss, as Easter struck a 92nd-minute equaliser as Redknapp's men lose ground on Burnley at the summit of the Championship.