The two-time runners-up are now one win away from a first trip to the new Wembley Stadium, which hosts the semi-finals on 10 and 11 April and the final on 15 May.
Derby might be kicking themselves for failing to hold on to their lead but must quickly re-focus back to the Championship, with the visit of Preston on Tuesday and Swansea on Saturday.
They Rams arrived with only one defeat in their previous seven matches, a sequence in which they saw off Doncaster, Nottingham Forest and Newcastle without conceding a goal.
So it was hardly surprising that they set the tempo early on, knocking the ball around with confidence, putting the visitors on the back foot and probing for a route to goal.
But Birmingham's defence, so solid all season, held firm and as the half wore on the Blues began to look dangerous.
Blues boss Alex McLeish had recalled Sebastian Larsson, James McFadden and Christian Benitez, thus sending out the same line-up which went unchanged for 12 matches before Wednesday's 2-0 defeat at West Ham.
A Lee Bowyer drive was gathered by Stephen Bywater at the second attempt before Jerome's curling effort was expertly palmed around the post for a corner.
Savage saw a 35-yard drive tipped over the bar late on
Birmingham looked the more likely scorers at that stage but Derby, inspired by former Blue Robbie Savage, were working hard to swing the momentum back in their direction.
Joe Hart saved well at his near post to deny Stephen Pearson and Rob Hulse arrived too fractionally too late after Kris Commons produced a fine solo run and cross from the right.
Derby almost took the lead shortly before half-time when Shaun Barker headed a Robbie Savage free-kick towards goal from about four yards, only for Hart to scramble the ball clear.
But the England goalkeeper arguably should have done better with the goal - McEveley riding a couple of challenges before unleashing a fierce left-footed shot which he could only push into the roof of the net.
Derby, winners in 1946 and bidding to reach the quarter-finals for the first time since 1996, went in search of a second but Birmingham kept their composure and repelled all that was thrown at them.
The Blues then set about restoring parity and, after substitute Phillips was superbly denied by Bywater, their reward arrived as Dann out-jumped Barker to power home McFadden's corner.
Birmingham were suddenly stirring and, urged on by their travelling support who heckled the superb Savage all match, they went for the winner.
Derby came under increasing pressure but looked like holding out for a replay until their hopes were dashed a minute into stoppage time.
Phillips controlled fellow-substitute Craig Gardner's scuffed free-kick and shot against the post. As the ball rolled along the goal line, Ridgewell arrived to stab it home and cue Birmingham's celebrations.
Savage went close with a 35-yard drive just before full-time but Derby boss Nigel Clough, whose father Brian Clough never won the FA Cup, will have to wait until next year for another crack of the whip.
Derby manager Nigel Clough told BBC Radio Derby: "I thought overall that we deserved at least a replay and we do feel slightly hard done by.
"Stephen Bywater did not have much to do and we created enough situations ourselves when 1-0 up and did not sit back.
"We were as comfortable as anyone can be when facing a team lying eighth in the Premier League but we were undone by two set plays. We look at the two goals and there was certainly an element of bad luck about them both."
Birmingham manager Alex McLeish: "We looked as if we were on our way out of the Cup. Derby troubled us before half-time and we didn't really get going until we made some changes.
"But once again the team demonstrated the fantastic determination they have produced time and time again this season.
"What we have to do now is maintain our form. If you get cocky and complacent the roof can cave in but we have got a lot of leaders in that dressing room."
90:00+8:08 The referee brings the game to a close.
90:00+4:11 Robbie Savage takes a shot. Save by Joe Hart. Corner taken by Kris Commons from the right by-line, Scott Dann manages to make
90:00+3:51 Craig Gardner gives away a free kick for an unfair challenge on Stephen Pearson. Stephen Bywater takes the direct free kick.
90:00+2:50 GOAL - Liam Ridgewell:Derby 1 - 2 Birmingham Liam Ridgewell grabs a goal from inside the six-yard box to the bottom left corner of the goal. Derby 1-2 Birmingham.
90:00+0:17 Michael Tonge is penalised for a handball. Scott Dann produces a shot on goal direct from the free kick, blocked by Jake Buxton.
Shot by Kevin Phillips from inside the penalty area hits the post.
89:30 Booking Craig Gardner is given a yellow card.
88:52 Unfair challenge on Robbie Savage by Kevin Phillips results in a free kick. Robbie Savage crosses the ball from the free kick
right-footed from left wing.
88:20 Corner taken right-footed by Michael Tonge to the near post, Craig Gardner makes a clearance.
87:35 Rob Hulse challenges Barry Ferguson unfairly and gives away a free kick. Joe Hart takes the direct free kick.
81:50 Substitution Craig Gardner on for James McFadden.
80:41 James McFadden takes a shot. Jake Buxton gets a block in.
80:07 Free kick awarded for a foul by Kris Commons on Lee Bowyer. Barry Ferguson takes the direct free kick.
78:49 A cross is delivered by Roger Johnson, Nicky Hunt manages to make a clearance.
78:16 Scott Dann delivers the ball, clearance made by James McEveley.
76:50 The referee blows for offside against Cameron Jerome. Direct free kick taken by Stephen Bywater.
75:36 Rob Hulse gives away a free kick for an unfair challenge on Roger Johnson. Direct free kick taken by Stephen Carr.
74:54 Cameron Jerome challenges James McEveley unfairly and gives away a free kick. Stephen Bywater takes the free kick.
74:04 Scott Dann gives away a free kick for an unfair challenge on Rob Hulse. Free kick taken by Kris Commons.
73:24 Foul by Rob Hulse on Roger Johnson, free kick awarded. Free kick taken by Joe Hart.
72:05 Assist on the goal came from James McFadden.
72:05 GOAL - Scott Dann:Derby 1 - 1 Birmingham Scott Dann scores a headed goal from close range. Derby 1-1 Birmingham.
71:49 Corner from the right by-line taken by James McFadden,
69:44 Inswinging corner taken left-footed by James McFadden from the right by-line, Liam Ridgewell has a headed effort from close
range and clears the bar.
69:29 Kevin Phillips takes a shot. Save by Stephen Bywater.
69:00 Unfair challenge on Lee Bowyer by Kris Commons results in a free kick. Free kick taken by Joe Hart.
67:52 Barry Ferguson takes a shot. Jake Buxton gets a block in.
67:13 Centre by Rob Hulse.
65:04 Unfair challenge on Nicky Hunt by Cameron Jerome results in a free kick. Direct free kick taken by Stephen Bywater.
63:52 Substitution Christian Benitez goes off and Kevin Phillips comes on.
63:52 Substitution Keith Fahey on for Sebastian Larsson.
61:47 Barry Ferguson takes the free kick.
61:47 Booking Robbie Savage is cautioned.
61:26 Free kick awarded for a foul by Robbie Savage on Christian Benitez.
58:59 Centre by Stephen Carr, blocked by Stephen Pearson. Corner taken left-footed by James McFadden from the right by-line, Jake
Buxton makes a clearance. Free kick awarded for an unfair challenge on Shaun Barker by James McFadden. Direct free kick taken
by Stephen Bywater.
58:27 Kris Commons takes a shot. Save made by Joe Hart.
57:50 Free kick awarded for a foul by Stephen Pearson on Lee Bowyer. Free kick taken by Barry Ferguson.
57:19 Kris Commons takes a shot. Save made by Joe Hart.
55:25 Free kick awarded for a foul by Stephen Pearson on Stephen Carr. Direct free kick taken by Joe Hart.
55:13 A cross is delivered by James McEveley, clearance made by Roger Johnson.
54:13 GOAL - James McEveley:Derby 1 - 0 Birmingham Goal scored by James McEveley from outside the area to the bottom right corner of the goal. Derby 1-0 Birmingham.
54:03 Michael Tonge produces a cross, Roger Johnson makes a clearance.
52:42 Cameron Jerome sends in a cross. The referee blows for offside. Stephen Bywater restarts play with the free kick.
52:01 The ball is swung over by Michael Tonge, clearance made by Roger Johnson.
Correction - 51:31 The ball is crossed by Nicky Hunt, clearance by Scott Dann.
51:31 Nicky Hunt produces a cross, Roger Johnson makes a clearance.
51:02 Rob Hulse fouled by Scott Dann, the ref awards a free kick. Direct free kick taken by Kris Commons.
50:05 Michael Tonge challenges Lee Bowyer unfairly and gives away a free kick. Free kick taken by Joe Hart.
47:20 The ball is delivered by James McEveley.
46:35 Michael Tonge sends in a cross, save by Joe Hart.
45:50 Robbie Savage is penalised for handball and concedes a free kick. Free kick taken by Sebastian Larsson. Sebastian Larsson
sends in a cross, Shaun Barker makes a clearance.
45:31 The assistant referee signals for offside against Rob Hulse. Free kick taken by Joe Hart.
45:01 The referee starts the second half.
45:00+1:11 The ref blows to end the first period.
45:00+0:11 Shot from just outside the box by Stephen Pearson goes over the crossbar.
44:06 Free kick awarded for a foul by Nicky Hunt on Lee Bowyer. Joe Hart takes the direct free kick.
40:09 Shaun Barker takes a shot. Liam Ridgewell makes a clearance. Centre by Michael Tonge, Headed effort from deep inside the penalty
area by Shaun Barker goes harmlessly over the crossbar.
39:27 Free kick awarded for a foul by Sebastian Larsson on Stephen Pearson. Direct free kick taken by Robbie Savage.
38:57 A cross is delivered by Kris Commons.
38:25 Free kick awarded for a foul by Stephen Pearson on Sebastian Larsson. Stephen Carr restarts play with the free kick.
37:19 Free kick awarded for an unfair challenge on Cameron Jerome by Shaun Barker. Free kick taken by Sebastian Larsson.
36:19 Effort from inside the penalty area by Stephen Pearson hits the post.
35:02 Scott Dann challenges Kris Commons unfairly and gives away a free kick. Robbie Savage takes the free kick, Effort from outside
the penalty area by James McEveley goes wide left of the goal.
33:46 Cameron Jerome takes a shot. Save made by Stephen Bywater. Corner from the right by-line taken by James McFadden, Close range
header by Roger Johnson goes over the crossbar.
32:43 The ball is sent over by Liam Ridgewell, Christian Benitez takes a shot. Stephen Bywater makes a save.
31:48 Lee Bowyer takes a shot. Stephen Bywater makes a save.
30:57 Stephen Pearson delivers the ball, blocked by Sebastian Larsson.
30:47 James McEveley produces a cross, Roger Johnson manages to make a clearance.
30:39 James McEveley sends in a cross, clearance by Roger Johnson.
28:06 Stephen Pearson produces a cross, Barry Ferguson makes a clearance.
27:37 Free kick awarded for a foul by Scott Dann on Rob Hulse. Nicky Hunt takes the direct free kick.
25:44 The assistant referee flags for offside against Christian Benitez. Robbie Savage restarts play with the free kick.
24:31 Kris Commons fouled by Roger Johnson, the ref awards a free kick. Kris Commons takes the free kick.
16:48 Cameron Jerome is ruled offside. Jake Buxton restarts play with the free kick.
16:07 Foul by Liam Ridgewell on Robbie Savage, free kick awarded. Direct free kick taken by Stephen Bywater.
13:26 James McEveley crosses the ball, clearance made by Lee Bowyer.
10:15 Corner taken by Sebastian Larsson.
4:05 Pass corner from right by-line crossed in by Kris Commons.
3:21 Corner taken by James McFadden, Paul Green makes a clearance.
2:19 A cross is delivered by Liam Ridgewell, clearance made by James McEveley.
0:00 The match begins.
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