Substitute Yousef Salech scored his first Cardiff City goal as the Bluebirds fought back from 2-0 down to earn a last-gasp point at The Den.
Millwall could scarcely have wished for a better start as Calum Scanlon lashed home after just 67 seconds with the first chance of the match.
The Lions were 2-0 ahead before the 20-minute mark after fabulous skill from Ra'ees Bangura-Williams created the chance for Casper de Noore to apply a finish from inside the penalty area.
Chris Willock curled home beautifully in first-half stoppage time to give Cardiff hope and the visitors were much improved after the interval, with Salech levelling with virtually the last kick of the match following a goalmouth scramble.
This was a game of real significance in the lower half of the Championship, with Millwall bidding to stay out of the relegation battle the Bluebirds have been in all season after an unprecedentedly poor start to their campaign.
Millwall boss Alex Neil made two changes to his side beaten 1-0 at Hull last time out, with 20-year old Bangura-Williams handed a first start as life after Romaine Esse begins at the New Den.
Cardiff came into this contest still looking over their shoulders anxiously at the relegation zone despite their sensational 3-0 win over bitter rivals Swansea City and general improved performances.
The win over the Swans was unquestionably the highlight of the season for the Bluebirds, however there was no prospect of naming a settled side with fitness and fatigue meaning boss Omer Riza made five changes to his starting side.
The New Den has not been a happy hunting ground for the Bluebirds, and they were bitten by the Lions within the opening 70 seconds as Scanlon pounced on a loose ball and broke clear on the left before hammering a precise finish into the roof of the net on the run.
The hosts were buoyed by the early strike and maintained their impressive tempo despite Cardiff seeing Yakou Meite fire over and Callum Robinson denied after his shot and turn.
The Lions have had creativity issues this term, not helped by the sale of England youth stand-out Esse to Crystal Palace, but the hosts may have found a new starboy in the guise of Bangura-Williams.
It was the 20-year old who deserves all the plaudits for Millwall's second as he provided a sensational assist, twisting and turning past Manolis Siopis with balletic precision before cutting the ball back for De Norre to apply a powerful finish.
It was a moment of real quality from Bangura-Williams in a first half where the hosts might even have gone in further ahead, but Japhet Tanganga headed over from a corner just before the break.
The miss from Tanganga proved to be a 'Sliding Doors' moment as Cardiff got back into the contest in stoppage time as Cardiff played on with two Millwall players down injured and Willock curled home beautifully from the edge of the penalty area.
The goal gave Cardiff hope and they started the brighter after the break as top scorer Callum Robinson's effort was saved by Lukas Jensen and Willock fizzed an effort from 25-yards just the wrong side of the post.
It was the visitors pushing and probing looking for a leveller, but Macaulay Langstaff's acrobatic effort did force Jak Alnwick to save.
Millwall sub Danny McNamara was stretchered off moments after coming on following a lengthy-layoff as Millwall saw would had been a comfortable evening become tight and tense.
Cardiff sub Anwar El Ghazi curled a freekick inches wide as the Bluebirds pushed for an equaliser and Jensen was forced to save at full stretch from Salech's header.
Salech was a big-money signing for Cardiff to try and end their woes in front of goal but he missed a huge chance in stoppage time as he headed wide, unmarked, from El Ghazi's inviting cross.
Salech would have had a sleepless night over his miss but got the last laugh as he found the net with virtually the last touch of the match following a scramble in the Millwall penalty area as he turned and fired home on the spin after Perry Ng's powerful run.
The goal ensures Cardiff remain outside the Championship relegation zone in 19th, while Millwall are three points and two places above the Bluebirds.
Millwall: Jensen, Cooper ©, Tanganga, Bryan, Langstaf, Leonard (McNamara 59’) (Crama 64’), Watmore (Mitchell 78’), Saville, De Norre, Bangura-Williams (Ivanovic 64’), Scanlon (Honeyman 78’). Subs not used: Roberts, Wallace, Hutchinson, Connolly.
Cardiff City: Alnwick ©, Rinomhota, Goutas, Daland (Fish 78’), Bagan, Chambers, Siopis (Ng 58’), R Colwill (Salech 69’), Willock (El Ghazi 78’), Méïté (Ashford 69’), Robinson. Subs not used: Horvath, Giles, Ralls, Pearce.