Smullen swooped for 1-2 in total, with his 27th-minute strike ultimately separating the sides and setting up a final date with Kerry.
The tie will be remembered though, for the storming finish by Kildare, who outscored Down 0-4 to 0-1 in the last quarter. Down had come in as favourites, with seven of the side that won the All-Ireland minor title back in 2005.
Paul McComiskey, a star man back then, was pivotal again and his three early points put Down into a 0-6 to 0-2 lead.
But they failed to build on their dominance, with several missed chances and four wides allowing Kildare right back into it when Smullen netted.
He held off his man on the left of goal and sneaked his shot home at the near post, with the excellent Alan Smith levelling things up soon after.
McComiskey’s fourth point of the half put Down 0-7 to 1-3 up at half time but it should have been more — and they were made to pay for their profligacy.
Having been level three times after the break, Kildare saved their best for late on, with a storming finish.
Substitute Joseph Cocoman put Kildare ahead with a booming kick and points from Smullen, Smith and Sean Fahy sealed a slightly flattering three -point win.
N McConnell; C Brophy, D Brennan, S Murphy; J Browne, G Whyte, T Byrne; M Waters (0-1), N Higgins; D Whyte, E Ó Flaitheartaigh (0-1), K Kelly; S Fahy (0-2), A Smith (0-3; 1f), G Smullen (1-2).
J Cocoman(0-1) for Kelly (44).
M McAllister; M Digney, C Murney, J Murphy; T Hanna, K Duffin, C Garvey; J Colgan, J Caldwell; C Maginn, K McKernan (0-1), P McComiskey (0-6; 3f); C Brannigan (0-3; 2f), C Magee, R Kelly.
B Rooney for Murphy (32), P Downey for Magee (47), C Killen for McGinn (51), K Anderson for Caldwell (53), J O’Reilly for Brannigan (58).
P McGovern (Galway).