Urwin's hat-trick of saves sent Chorley through after an epic clash at Victory Park.
After Alex Newby and Adam Blakeman missed their penalties Urwin saved Jake Moult and Shan Densmore's spot-kicks to keep it locked at 0-0.
Chorley's Courtney Meppen-Walter beat Anthony Thompson and then Urwin saved James Poole's penalty.
Louis Almond made it 2-0 but Urwin was finally beaten by Max Harrop.
The pressure fell on Wilson's shoulders but he was as cool as a cucumber to make it 3-1 and book Chorley's place in the final against Spennymoor Town.
The Magpies found themselves a goal down after Joshua Hancock netted in the 67th minute.
But he turned from hero to zero as he was shown a straight red card for a late challenge on Marcus Carver.
Despite Altrincham defending in valiantly Josh Wilson popped up with an 82nd minute header.
The Magpies roared on but despite throwing everything but the kitchen sink at their visitors they could not find a way through in normal or extra time.
That sent the tie to penalties and set the stage for Urwin.
Thompson had been Altrincham's saviour throughout the game but it was the opposite number one that was left beaming with delight at the end of the day.
A minute's applause rang around the ground before kick-off in tribute to club legend Danny Mill's wife Claire who lost her battle with cancer last week.
The Magpies also wore black armbands in tribute for this play-off semi-final.
Altrincham had battled their way through extra-time and penalties against Blyth Spartans to secure this date while Chorley did not need an eliminator having finished second in the table.
That gave them home advantage and despite having had a midweek game Altrincham looked fresh from the kick-off.
Boss Jamie Vermiglio was boosted by the return of key trio skipper Andy Teague, striker Josh Wilson and wing-back Matt Challoner from injury.
But despite that boost the Magpies started nervously.
Matty Urwin and Scott Leather tried to play the ball out from the back with Courtney Meppen-Walter finding himself in trouble.
That lead to an early chance for Altrincham's danger man John Johnston.
But his cross flew off target.
He would be a thorn in Chorley's side as the half progressed as he targeted left wing-back Adam Blakeman who tired as the half went on.
Jake Cottrell sat deeper in front of the back three to try and stop Altrincham's midweek hero Joshua Hancock pulling the strings.
And Cottrell did a sterling job in the first 45 with some key tackles, blocks and interceptions.
Wilson and Carver worked tirelessly up top pressing but they could not capitalise on an Altrincham mistake.
The referee could have littered the pitch with yellow cards and should have carded Joshua Hancock for a strong challenge on Teague.
The Magpies went route one from the resulting set-piece with a flick-on eventually falling to Anthony Thompson with no Magpies gambling in the box.
Chorley nearly took the lead just moments later though.
Blakeman's corner from the right was inch-perfect and flew into the path of Teague.
The Chorley skipper thundered the ball towards goal but Thompson was up to the challenge with a fine reflex stop to bat away the ball and collect at the second attempt.
Carver threatened but he took too much time after finding space on the right and the chance was gone.
Altrincham's main asset was pace and they looked to sting on the counter through Johnston.
He made his way to the back line but he was thwarted by a recovering Blakeman as Chorley managed to contain the opposition's threat.
Though they were caught napping soon after.
Jordan Hulme spun in the box and his effort crashed into the side netting.
The referee Elliot Swallow ruled that Matty Urwin had got a touch though that was debatable.
Again the referee waved away an Altrincham penalty claim as Jake Moult went to ground in Chorley's box but the referee did not book the Altrincham man for simulation.
The Newby brothers could not find space and when they did get the ball they wanted a touch too many.
Though that would change after the break.
The tackles flew in from both sides and tempers threatened to boil over.
Blakeman and Wilson combined to tee up Elliot Newby but his effort was blocked,.
Though again the visitors targeted Chorley's left flank with Johnston feeding Hulme, he pulled the ball back to Hancock who fired over when he should have hit the target.
Both sides continued to cancel each other out as they approached the break but it was a fine advert for National League North football despite the lack of goals.
Johnston handled in the box and again the referee did not reach for his pocket but up the other end Chorley just failed to pounce on a Blakeman set-piece.
Whatever Vermiglio said at half-time worked as the Magpies flew out of the traps and found another level.
But they could not get that all important goal their start deserved.
Elliot Newby was so close to giving the Magpies the lead just minutes into the second 45 as his curling effort rolled towards the corner but kissed the post with Carver unable to convert the rebound.
Carver's effort is batted away and just cannot be converted by Alex Newby or Wilson with Altrincham at sea.
Elliot Newby again found space to attack and his effort was just tipped away by Thompson.
The Magpies had managed to get the Newby's on the ball in the second 45 and their attack looked sharper.
The home faithful just hoped they would not be left to rue those missed chances.
Hulme was forced off through injury with James Poole entering the fray but he fired over after being given too much space outside the box.
The Magpies were to pay for those missed chances.
Joshua Hancock broke the offside trap as Chorley were felled on a corner and he was composed to slide the ball under Urwin in the 67th minute.
The Magpies pushed for a leveller but the ball just would not fall for them.
Chorley were given a glimmer of hope as Altrincham's goal hero Hancock was shown a straight red card for a late challenge on Marcus Carver in the 72nd minute
Could Chorley make their man advantage pay?
Vermiglio sent on last weekend's hat-trick hero Louis Almond replacing Elliot Newby as the Magpies went with three up front to try and get that leveller.
But that saw gaps for Altrincham too with Yusifu Ceesay threatening to sprint clear before Cottrell recovered and Urwin mopped up.
Almond fresh and hungry off the bench found time to whip his foot around the ball in the box but his effort flew comfortably to Thompson as the Magpies camped in Altrincham's half.
Vermiglio threw O'Keefe on for Matt Challoner but the Magpies still could not find a way to break down Altrincham's defence.
That was until the 82nd minute when the Magpies finally beat Thompson.
A Blakeman long throw from the right was flicked on by Teague and looked to be flying home but poacher Josh Wilson popped up at the back stick to ensure the ball crossed the line.
The Magpies pushed on but again Altrincham held firm with all 10 men behind the ball asking Chorley to attack.
Almond found a pocket and rolled the ball to Cottrell on the edge of the box, his effort was charged down and Altrincham countered.
Fortunately Scott Leather was on hand to thwart the visitors.
The Magpies had a late corner but could not use it and the tie went to extra time.
The other semi-final also went to extra time as Spennymoor and Brackley finished normal time with a goalless draw.
The Magpies huddled before the game resumed and they attacked from the off.
Wilson went closest but again his header was collected by Thompson.
They nearly made it 2-1 in glorious fashion as Alex Newby cut Altrincham open like a hot knife through butter.
But his eventual strike was just stopped by Thompson.
With that extra man advantage Alex Newby was able to get a grip on the game and it he was again the architect of another chance.
The Magpies tried something outside of the box from a set-piece as Blakeman played it short to Alex Newby, he teed-up Scott Leather in acres of space but his thunderous effort just flew wide of the right stick.
Again the Magpies attacked, this time Meppen-Walter's cross whirled through the air to O'Keefe.
Time seemed to stand still as it flew towards goal but somehow Thompson got across to pluck the ball out of fresh air.
It was one way traffic and the Magpies did not want the half to end.
That was until skipper Teague went down with cramp after putting his body on the line once again to go for a corner.
The Magpies came out fighting again in the second half of extra time.
They were queuing up but could not get a shot away despite O'Keefe just needing to pull the trigger.
He opted to pull the ball back to Alex Newby but his curling effort eventually dropped to Thompson.
The Magpies took a patient approach as they camped in Altrincham's half.
Meppen Walter's header was cleared off the line by Poole and Wilson's first-time effort clattered into the advertising boards.
But despite their best efforts the Magpies could not find a way through.
They were throwing everything but the kitchen sink at their visitors but Almond fired wide of the left stick after Wilson teed him up.
Altrincham won a set-piece but the Magpies defended valiantly and the tie was destined for penalty kicks.
Chorley: Urwin, Challoner (O'Keefe, 80), Teague, Leather, Meppen-Walter, Blakeman, Cottrell, A Newby, E Newby (Almond, 75), Wilson, Carver (Tuton, 119). Subs not used: Anson, Tuton, Jordan.
Altrincham: Thompson, Densmore, Hampson, Jones, Hannigan, Moult, Johnston, Williams (Richman, 65), Hulme (Poole, 54) Hancock, Ceesay (Harrop, 95). Subs not used: Poole, Harrison, Richman, Wynne.