Whittingham's left-foot curler from 25-yards beat Leicester keeper Chris Weale before going in off his near post.
Michael Chopra had earlier hit a post for Cardiff while Leicester's Matt Fryatt squandered a late chance.
Fryatt's missed opportunity was a host of chances for the rampant hosts in a frantic final 10 minutes as Leicester had two penalty appeals rejected and keeper David Marshall made two top-class saves.
Substitute Fryatt sprinted through-one-on-one as he threatened to capitalise on a tangle between Darcy Blake and Kevin McNaughton but Marshall remained on his feet and beat out the Foxes top scorer's shot.
Cardiff's goal led a charmed life late on as Leicester's pressure intensified as Paul Gallagher curled a right-foot shot wide of Marshall's far post before the same man had a shot blocked in time added on.
Scottish international Marshall had to be at his best again with virtually the last kick of the game as he somehow clawed Alex Bruce's close-range bullet header - from Gallagher's in-swinging left-wing corner - out from point-blank range.
While Marshall was Cardiff's unsung hero at one end, Whittingham stole the show at the other as the Bluebirds winger fired in a goal-of-the-season contender when it really mattered.
Whittingham's desire has often been criticised in previous seasons but his eye for a goal - and timing - cannot be questioned as his beautiful left-foot strike has fired Cardiff to the brink of their first Championship play-off final.
A Wembley showpiece with either Blackpool or Nottingham Forest on 22 May and a shot at money-spinning promotion to the Premier League - potentially banking a £90m jackpot for the debt-ridden Welsh club - is now more than just a dream.
Minute by minute (read from bottom up)
Full Time
90:00+6:12 The referee blows for the end of the match.
90:00+5:48 Corner taken right-footed by Paul Gallagher from the left by-line to the near post, Kevin McNaughton makes a clearance.
90:00+5:06 Corner taken by Paul Gallagher from the left by-line played to the near post, Bruno Berner takes a shot. Save made by David
Marshall.
90:00+5:06 Booking David Marshall receives a caution.
90:00+4:38 Dany N'Guessan produces a cross.
90:00+3:33 Foul by Yann Kermorgant on Joe Ledley, free kick awarded. Free kick taken by Mark Kennedy.
90:00+3:11 Corner taken right-footed by Paul Gallagher.
90:00+2:53 Matt Fryatt takes a shot. Save by David Marshall.
90:00+2:26 Yann Kermorgant takes a shot. Mark Kennedy gets a block in.
90:00+0:35 Ross McCormack takes a shot. Chris Weale makes a save.
89:33 Substitution Ross McCormack on for Michael Chopra.
89:09 Centre by Dany N'Guessan.
Correction - 87:38 Free kick awarded for an unfair challenge on Yann Kermorgant by Jay Bothroyd. Free kick crossed right-footed by Paul Gallagher,
clearance made by Joe Ledley.
87:38 Free kick awarded for an unfair challenge on Yann Kermorgant by Jay Bothroyd. Free kick crossed right-footed by Paul Gallagher,
Mark Hudson makes a clearance.
85:57 Paul Gallagher has an effort at goal from the edge of the area that misses to the right of the target.
Correction - 84:53 Inswinging corner taken from the left by-line by Paul Gallagher, Alex Bruce produces a header from inside the area that goes
over the crossbar.
Correction - 84:53 Substitution Dany N'Guessan replaces Richie Wellens.
84:53 Inswinging corner taken from the left by-line by Paul Gallagher.
84:53 Substitution Dany N'Guessan is brought on as a substitute for Richie Wellens.
83:04 Matt Fryatt crosses the ball.
82:33 Matt Fryatt takes a shot. Save by David Marshall.
82:01 Inswinging corner taken by Peter Whittingham from the right by-line, save by Chris Weale.
Correction - 81:44 Jay Bothroyd produces a cross, Alex Bruce makes a clearance.
81:44 Jay Bothroyd produces a cross, clearance by Bruno Berner.
80:58 Alex Bruce challenges Michael Chopra unfairly and gives away a free kick. Peter Whittingham delivers the ball from the free
kick left-footed from left wing.
Correction - 79:29 Richie Wellens concedes a free kick for a foul on Michael Chopra. Shot comes in from Peter Whittingham from the free kick,
Richie Wellens gets a block in.
79:29 Free kick awarded for a foul by Richie Wellens on Michael Chopra. Shot comes in from Peter Whittingham from the free kick,
Andy King gets a block in.
79:10 Booking Mark Kennedy goes into the book.
77:04 GOAL - Peter Whittingham:Leicester 0 - 1 Cardiff Free kick scored by Peter Whittingham from outside the penalty area to the top right corner of the goal. Leicester 0-1 Cardiff.
76:41 Unfair challenge on Michael Chopra by Yann Kermorgant results in a free kick.
76:03 Michael Chopra takes a shot. Bruno Berner gets a block in.
75:38 Corner taken left-footed by Peter Whittingham, Jack Hobbs makes a clearance.
75:22 Substitution (Leicester) makes a substitution, with Yann Kermorgant coming on for Martyn Waghorn.
75:08 Joe Ledley delivers the ball, clearance by Alex Bruce.
73:45 Peter Whittingham crosses the ball from the free kick left-footed from left wing, Bruno Berner manages to make a clearance.
73:45 Substitution Kelvin Etuhu is brought on as a substitute for Chris Burke.
73:45 Free kick awarded for an unfair challenge on Joe Ledley by Richie Wellens.
72:08 Corner from the left by-line taken by Paul Gallagher, Mark Hudson makes a clearance.
71:52 The ball is delivered by Bruno Berner, blocked by Mark Hudson.
67:45 Centre by Richie Wellens, save by David Marshall.
66:48 Centre by Jay Bothroyd.
64:54 Unfair challenge on Bruno Berner by Jay Bothroyd results in a free kick. Chris Weale restarts play with the free kick.
63:20 The ball is sent over by Richie Wellens, Kevin McNaughton manages to make a clearance.
61:24 Substitution Matt Fryatt replaces Lloyd Dyer.
61:04 Jay Bothroyd is penalised for handball and concedes a free kick. Jay Spearing restarts play with the free kick.
59:05 Lloyd Dyer produces a left-footed shot from the edge of the area and misses to the left of the goal.
58:37 Martyn Waghorn takes a shot. Save made by David Marshall.
57:28 Free kick awarded for an unfair challenge on Martyn Waghorn by Darcy Blake. Strike on goal comes in from Richie Wellens from
the free kick.
56:03 Paul Gallagher takes a inswinging corner to the near post, Jay Bothroyd makes a clearance.
55:14 Chris Weale restarts play with the free kick.
55:14 Booking Jay Bothroyd is cautioned by the ref for unsporting behaviour.
54:48 Jay Bothroyd challenges Jack Hobbs unfairly and gives away a free kick.
52:58 Richie Wellens has an effort at goal from outside the area missing to the wide right of the target.
52:08 Bruno Berner crosses the ball, blocked by Kevin McNaughton.
51:13 Martyn Waghorn fouled by Mark Hudson, the ref awards a free kick. Richie Wellens takes the free kick.
49:24 The assistant referee signals for offside against Martyn Waghorn. Free kick taken by David Marshall.
48:28 A cross is delivered by Kevin McNaughton, Header from deep inside the penalty area by Jay Bothroyd goes harmlessly over the
crossbar.
Correction - 47:36 Shot on goal by Stephen McPhail from just inside the area goes over the target.
47:36 Effort on goal by Stephen McPhail from just inside the penalty area goes harmlessly over the target.
46:59 Corner taken right-footed by Chris Burke, Jack Hobbs makes a clearance.
45:46 Effort on goal by Jay Spearing from long range goes harmlessly over the target.
45:01 The second half begins.
Half Time
45:00+1:20 The referee blows for half time.
45:00+0:46 A cross is delivered by Nolberto Solano, Andy King challenges Mark Hudson unfairly and gives away a free kick. David Marshall
takes the direct free kick.
44:58 Martyn Waghorn takes a shot. Blocked by Darcy Blake.
44:43 Paul Gallagher delivers the ball, Mark Hudson makes a clearance.
42:07 The free kick is swung in left-footed by Peter Whittingham.
42:07 Booking The referee shows Jack Hobbs a yellow card for unsporting behaviour.
42:00 Free kick awarded for a foul by Jack Hobbs on Jay Bothroyd.
40:40 Mark Kennedy delivers the ball, Jack Hobbs manages to make a clearance.
39:15 Michael Chopra produces a right-footed shot from inside the six-yard box and bounces off the post.
38:27 Free kick awarded for a foul by Jay Bothroyd on Richie Wellens. Bruno Berner takes the direct free kick.
37:26 The ball is swung over by Nolberto Solano, blocked by Joe Ledley.
36:05 A cross is delivered by Chris Burke, clearance by Alex Bruce.
35:52 A cross is delivered by Jay Bothroyd, Chris Weale makes a brilliant save.
35:22 Inswinging corner taken by Peter Whittingham, Jack Hobbs makes a clearance.
35:12 The ball is delivered by Michael Chopra, Jack Hobbs gets a block in.
34:20 Michael Chopra takes a shot. Jack Hobbs gets a block in.
33:28 Alex Bruce fouled by Jay Bothroyd, the ref awards a free kick. Alex Bruce restarts play with the free kick.
32:40 Darcy Blake has an effort at goal. Save by Chris Weale.
31:54 Corner taken left-footed by Peter Whittingham to the near post, Jack Hobbs manages to make a clearance.
31:44 Michael Chopra sends in a cross, Alex Bruce manages to make a clearance.
31:20 Martyn Waghorn is adjudged to have handled the ball. Direct free kick taken by David Marshall.
29:30 The referee blows for offside against Michael Chopra. Indirect free kick taken by Chris Weale.
26:46 Richie Wellens gives away a free kick for an unfair challenge on Jay Bothroyd. Chris Burke takes the direct free kick.
Correction - 25:39 Martyn Waghorn gives away a free kick for an unfair challenge on Stephen McPhail. David Marshall takes the direct free kick.
25:39 Martyn Waghorn gives away a free kick for an unfair challenge on Stephen McPhail. David Marshall takes the direct free kick,
25:02 Jay Bothroyd challenges Richie Wellens unfairly and gives away a free kick. Chris Weale takes the direct free kick.
24:35 Martyn Waghorn gives away a free kick for an unfair challenge on Darcy Blake. David Marshall restarts play with the free kick.
21:04 Michael Chopra has an effort at goal from deep inside the area missing to the left of the goal.
20:37 Handball by Jay Bothroyd. Jack Hobbs takes the direct free kick.
19:37 Inswinging corner taken by Martyn Waghorn from the right by-line, save made by David Marshall.
19:26 The ball is delivered by Lloyd Dyer, Mark Kennedy makes a clearance.
19:13 The ball is crossed by Bruno Berner, Mark Hudson makes a clearance.
17:13 Centre by Stephen McPhail, Jay Bothroyd gives away a free kick for an unfair challenge on Alex Bruce. Chris Weale restarts
play with the free kick.
16:02 Unfair challenge on Stephen McPhail by Martyn Waghorn results in a free kick. Direct free kick taken by David Marshall.
Correction - 14:07 Martyn Waghorn fouled by Darcy Blake, the ref awards a free kick. Strike on goal comes in from Nolberto Solano from the free
kick, David Marshall makes a comfortable save.
14:07 Unfair challenge on Martyn Waghorn by Darcy Blake results in a free kick. Nolberto Solano has a direct shot on goal from the
free kick, David Marshall makes a save.
11:59 Inswinging corner taken from the right by-line by Martyn Waghorn, Joe Ledley manages to make a clearance.
9:44 Effort from outside the area by Jay Bothroyd goes wide right of the goal.
8:59 Chris Burke delivers the ball, save made by Chris Weale.
8:31 Foul by Paul Gallagher on David Marshall, free kick awarded. David Marshall restarts play with the free kick.
Correction - 7:56 Corner taken left-footed by Martyn Waghorn from the right by-line, David Marshall makes a save.
7:56 Corner taken by Martyn Waghorn, save by David Marshall.
7:18 Inswinging corner taken by Martyn Waghorn from the right by-line, Mark Hudson makes a clearance.
6:31 Stephen McPhail challenges Richie Wellens unfairly and gives away a free kick. Paul Gallagher shoots direct from the free
kick, blocked by Jay Bothroyd.
6:05 Richie Wellens takes a shot. Mark Hudson gets a block in.
4:41 The ball is sent over by Kevin McNaughton, clearance made by Jack Hobbs.
4:24 The ball is swung over by Peter Whittingham, clearance by Jack Hobbs.
3:36 Chris Burke sends in a cross.
2:55 Shot by Peter Whittingham from 18 yards. Blocked by Paul Gallagher.
2:19 Chris Burke takes a inswinging corner from the left by-line played to the near post, clearance made by Jack Hobbs.
1:59 Jay Bothroyd takes a shot. Blocked by Alex Bruce.
0:13 Jay Bothroyd concedes a free kick for a foul on Jack Hobbs. Free kick taken by Jack Hobbs.
0:00 The match gets underway.
Live text and data provided by The Press Association.
Source: BBC Sport
Source: BBC Sport