A crowd of almost 8,000 packed into Elland Road for the play-off final with plenty turned away at the turnstiles ahead of the Bank Holiday showdown.
Jack Clarke’s penalty won the tie after the game finished goalless at the end of extra-time. Leeds were indebted to two saves from goalkeeper Kamil Miazek in the shoot-out.
United welcomed the Blues with bragging rights on the line as the pair locking horns for a third time this season.
Carlos Corberan selected a strong line-up for the clash, including 19-goal striker Ryan Edmondson.
Leeds dominated early proceedings but were kept largely at bay by the visitors in the first half.
Jack Clarke went close as did Ryan Edmondson but the game remained goalless at the half-time whistle.
United, though, started the second half with a spring in their step as the crowd rallied them on.
Pascal Struijk powered a header goalwards from a Mateusz Bogusz free-kick but Zack Jeacock was equal to it in the visiting goal.
United pushed on but it was City who looked the likelier of the two teams as full-time approached.
Substitute Olly McCoy forced Miazek into action before Jamie Shackleton produced a fine last-ditch tackle to deny the winger just moments later.
The two sides failed to be separated as United entered extra-time for the second game running in the play-offs.
It was a tight affair but City full-back Remeao Hutton almost broke the deadlock with a thunderous effort which smashed against Miazek's crossbar.
Bogusz then returned the favour as his free-kick left Jeacock rooted to the spot at the ball cannoned off a post.
Edmondson was on hand to tap in the rebound from close-range but the linesman's flag was raised to cut short the celebrations.
Penalties were needed and Birmingham took an immediate advantage when Bogusz struck the woodwork but Miazek saved from Joe Redmond and Caolon Boyd-Munce to hand Clarke the chance to seal it.
The winger powered home from 12-yards to spark wild celebrations as United were crowned PDL champions.