Kanu was Portsmouth's hit-and-miss hero again as his first-half goal won the FA Cup for Harry Redknapp's team, who had just enough Premier League pedigree to see off Championship outsiders Cardiff 1-0.
But Kanu and Pompey's joy was despair for the Welsh club's Finnish goalkeeper Peter Enckelman, who palmed a low cross by John Utaka straight to the long-limbed striker, who half-volleyed it into the net from about three yards.
Kanu scored only his seventh goal of the season, and the first since his semi-final winner against West Brom last month, which was a similar close-range effort from a keeper's parry.
And although he squandered two other marvellous opportunities in a match that failed to produce any great exciting football, he was granted a standing ovation by the delirious Pompey fans when replaced by Milan Baros for the final few minutes.
It was Pompey's who 11th 1-0 win of the season, earning them a place in next season's UEFA Cup - their first European adventure to go with their first FA Cup triumph for 69 years.
Cardiff, the fifth team from football's second tier to meet Pompey in the FA Cup this season, rarely threatened after an opening burst and Portsmouth saw out the game with solid defending in front of largely under-employed goalkeeper David James.
James, back from a calf injury, had to be alert to deny the dangerous Paul Parry twice in the opening 12 minutes as Cardiff settled the quicker despite Enckelman having to deal with a nasty deflection off Muntari's driven 40-yard free kick in Pompey's first attack.
The England keeper had to come off his line after Peter Whittingham's ball in from the right caught Sol Campbell out of position and sailed over his head just too far in front of the striker.
And when Parry, who missed the semi-final through injury, got himself on the end of Joe Ledley's neat flick which again left Campbell gasping, James had to put his 6ft 3in frame in the way once more and the shot appeared to spin away off his right thigh.
But with Sulley Muntari and Lassana Diarra finally starting to buzz in midfield and Pedro Mendes - a surprise choice in front of big Papa Bouba Diop - spraying smart passes, Pompey began to impose themselves.
After 21 minutes, Herman Hreidarsson made a muscular run down the left to feed Muntari, who turned the ball cleverly inside to Kanu.
The striker had the fans on the edge of their seats with a delightful shimmy that left Enckelman in a helpless heap, but with the goal gaping, he somehow managed to roll his shot against the outside of a post.
Pompey's 4-5-1 formation was a puzzle. Diarra was often further forward than designated lone-striker Kanu and shot wide with a snap effort from 20 yards.
But they had had problems to sort out at the back against Tony Capaldi's monster throw-ins and the crisp delivery of Stephen McPhail and Ledley.
When Glen Johnson briefly went missing at right back, McPhail's pass gave Parry acres of room just outside the area on left but instead of shooting as he entered the box, he tried to cut it back for Kevin McNaughton, who arrived too late on the far post.
Then Cardiff's Roger Johnson came up from the back to outjump Sylvain Distin and Campbell and nod Whittingham's free kick from the left just over.
But after 37 minutes, Pompey were handed their gift when John Utaka, who had struggled to get into the game out on the right, took on Capaldi and found just enough space to cross low into the six yard box.
Enckelman made a hash of collecting the ball, palming it straight to Kanu, who joyfully tucked the rebound home.
It was hard to see Cardiff coming back after that, even though they had the ball in the net just before half-time when, in the manic scramble that followed a half-cleared corner, Dutch centre-half Glenn Loovens charged the ball down with his upper arm before lobbing precisely over James.
And they should have been two down six minutes into the second half when Kanu spurned another golden chance, scuffing his shot from a central position inside the area after Kranjcar's spectacular backheeled pass put Diarra clear down the left to provide the cross.
Kanu claimed his effort struck the arm of defender Johnson but referee Mike Dean, who had sound game, ruled it accidental and gave a corner.
Cardiff gave it a real go right to the end, sending on Aaron Ramsay, 17, to become the second youngest player to appear in an FA Cup final but they could not break through Pompey's redoubled defence.