The Lions, who finished fifth after Bristol Rovers and Stoke City lost on Saturday, clinched their play-off place with a priceless early goal against Oxford by Neil Harris, taking his tally to 25 for the season.
Despite a few nervy moments towards the end more than 1,000 jubilant and well- behaved fans swarmed onto the pitch in celebration.
And Millwall also had something to celebrate in midweek when their reserves clinched the Avon Insurance Combination Championship with a 1-0 win over Wycombe at the New Den where Paul Shaw scored the all-important winner and Nigel Spink saved a penalty.
But the bad news for the Lions is that they could be without Lucas Neill and Tim Cahill for the early part of next season after the Australian Olympic squad's decision to play in a four-team tournament before the Sydney games get underway.
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