A tightly contested semi final at Haywards Heath was ultimately settled by two superb strikes from Joseph, the first of which was a goal of the season contender.
Having already sealed promotion back to the top flight on Saturday, United will now meet Wick & Barnham in the final on May 4 as the top two sides in the league do battle for the trophy.
United boss Nigel Geary was delighted as his side booked a final spot.
He said: ““I couldn’t be prouder of the team, they’ve been phenomenal all season long and not many sides win the double in the same season so the final will be a great occasion.
“We knew what Mile Oak’s game plan was going to be having already played them twice, they set up to stop us playing and to their credit they did that well, but we were the better side and deserved to win.
“This wasn’t our best performance of the season, but Wayne Joseph is probably the best winger in the league, individuals can win you games and his two goals have done that for us tonight.
“It doesn’t happen too often that the two best sides in the league actually reach the final, we have beaten Wick twice and they have won against us at their place so it’s going to be another tough game but to win the double would be fantastic.”
Jamie Watts had an early effort well saved by former Oak keeper Matt Grimstone, whilst at the other end Jacob Schilt’s sighter dropped narrowly over the bar after he was played in by Joseph.
United took the lead on 17 minutes as Joseph picked up the ball near the half-way line, dribbled past three Oak players, cut inside and unleashed an unstoppable left footed drive into the top corner from 20 yards.
Having burst into life, the next goal in the game was crucial and Oak’s equaliser came just before the hour mark.
Ransom’s rasper from Lloyd Skinner’s pass was tipped past the post by Grimstone and from the resultant corner, United failed to clear their lines and Watts bundled in from close range.
Parity did not last long, though, as Nigel Geary’s side restored their lead within a minute as Joseph fired in a finish from close range after Paul Elliott put him through.
Aaron Hasan hit the top of the bar with a header and Joseph went close to his hat-trick as United looked to kill off the contest.
MILE OAK: Stenning; J Hall, MacDonald, Turley-George, Marsh; Skinner, Garrod, French, Ransom; Hudson, Watts. Subs: Pulling (Garrod 30), Gillingham (Stenning 67), Koroma (MacDonald 78), Brown
UNITED: Grimstone; Gilmour, Hasan, Philpott, Taylor; Edwards, Hunt; Schilt, P Elliott, Joseph; Blundell. Subs: O’Regan (Elliott 74), MacNeill (Schilt 80), Stideford (Hunt 83), Wotherspoon, Evans.