Gary O'Neil gave Cardiff the lead five minutes before the break with a lobbed right-foot shot from 18 yards out.
Jo Tessem struck from close range soon after the break after being set up by Neil Harris, before Alan Lee's header from a Joe Ledley corner made it 2-1.
Millwall fired straight back, Harris meeting Alan Dunne's cross with a powerful header into the bottom right of Tony Warner's goal.
Millwall assistant manager Ray Wilkins:
"It would have been pleasant to
take all three points and have six points for the week after Tuesday's win over Gillingham.
"We showed lots of resilience but we are not too happy with the goals we conceded - although they got away with their second goal.
"The referee will blow for a free-kick in 99.9% of those situations but he made the decision that he thought was right and we have to live with that."
Cardiff manager Lennie Lawrence:
"I'm very happy with the performance but a little disappointed that - having gone into the lead twice - we didn't win.
"But let's be honest - it was a great game of football and good advert for the Championship.
"They came out a different outfit in the second half and will say a draw was fair and I say we shaded it and possibly should have won."
Millwall: Stack, Muscat, Lawrence, Ward, Livermore, Dunne,
Wise, Morris, Simpson (Harris 45), Tessem, Hayles.
Subs Not Used: Marshall, Serioux, McCammon, Elliott.
Booked: Morris, Harris, Muscat.
Goals: Tessem 47, Harris 74.
Cardiff: Warner, Williams, Gabbidon, Vidmar, Barker, McAnuff,
O'Neil, Kavanagh, Ledley, Thorne, Lee.
Subs Not Used: Collins, Bullock, Alexander, Jerome, Weston.
Booked: Kavanagh, Vidmar, Lee.
Goals: O'Neil 40, Lee 68.
Ref: H Webb (S Yorkshire).