Pellegrini's side looked confident of adding to their lead but against the run of play Valencia restored parity. Hugo Duro's run was fed by Ilaix Moriba and the on-loan Getafe forward made no mistake, dinking it over the onrushing Claudio Bravo with aplomb.
Betis wideman Sergio Canales struck the woodwork on the stroke of half-time, but Los Che applied pressure on the opposition rearguard in the opening stages of the second half. Carlos Soler ought to have put them ahead but somehow contrived to miss the target with the goal gaping.
With the clock ticking down in normal time, Juanmi had a golden chance to restore Betis' lead when he was set free on goal but couldn't find a way past Mamardashvili. He would go even closer, his low shot from the edge of the box denied only by the post.
Betis continued to knock on the door without reward. Again, Mamardashvili was Valencia's hero as he repelled Nabil Fekir when through on goal.
Both sides continued to trade blows, but a winner couldn't be found in 90 minutes and so the game proceeded into extra-time where chances were few and far between and the inevitable penalty shoot-out followed.
The first six penalties were perfect but at 3-3, Yunus Musah lifted his spot-kick over the bar. Betis took full advantage and there were wild scenes of jubilation when Miranda netted the decisive kick.
Betis players celebrate at the end of the Spanish Copa del Rey (King's Cup) final football match between Real Betis and Valencia CF at La Cartuja Stadium in Seville, on April 23, 2022
Image credit: Getty Images
TALKING POINT - A special night in the history of Real Betis
For just the third time in the club's history, Real Betis have been crowned winners of the Copa del Rey. The Green and Whites are pushing for a top-four finish in La Liga, but a spot in the Europa League at least has now been guaranteed after a night that will go down in their folklore.
Betis first won this trophy in 1977 and Joaquin was a key member of the team that won the 2005 edition. He won it again with Valencia in 2008 and tonight, some 14 years later, the 40-year-old came off the bench to help his beloved Betis claim the cup. It's some way to bow out after 21 consecutive years of top-flight football as he looks forward to retirement at the end of the season.
It's a similarly remarkable story for Pellegrini, who adds another trophy to his CV in his 34th year in management. Unlike Joaquin, however, the 68-year-old has no plans to call it a day just yet. And with Champions League football potentially to look forward to next season, who can blame him?
11' - GOAL! REAL BETIS 1-0 VALENCIA (BORJA IGLESIAS): Betis draw first blood! What a moment for the club! Fekir spreads the ball to Bellerin down the right flank and his cross is inch-perfect for Iglesias who attacks with his head and leaves the goalkeeper with no chance!
30' - GOAL! REAL BETIS 1-1 VALENCIA (HUGO DURO): Duro restores parity with a cool finish! There's a huge hole in Betis' midfield as Moriba drives Valencia forward. His threaded through ball finds the run of Duro and, cool as you like, he dinks it over the onrushing Bravo to level the scores!
SHOOT-OUT - MISS! VALENCIA 3-3 BETIS: Musah didn't look confident in his approach and he blazes his penalty over the bar! Advantage Betis?!
SHOOT-OUT - GOAL! BETIS 5-4 VALENCIA - BETIS WIN THE COPA DEL REY! Miranda scores the winning penalty and, for the first time since 2005, Betis are the Copa del Rey champions!
Real Betis have won their second title in the 21st century (excluding second tier), coming both in Copa del Rey and with Joaquin Sanchez playing the final, winning both titles in the interval of 16 years, 10 months and 12 days (2005-2022).
Betis have become the only team to have won two Copa del Rey titles on penalty shootouts: against Athletic at the Vicente Calderón in 1977 and today against Valencia at La Cartuja