The 26-year-old centre-back netted with a superb 69th-minute shot into the bottom right corner after Poland had midfielder Grzegorz Krychowiak sent off in the 62nd minute for a second yellow card.
Slovakia took an 18th-minute lead out of the blue through an own goal by Poland goalkeeper Wojciech Szczesny before Karol Linetty equalised in the 46th.
🇸🇰 Škriniar’s 3 international goals have come in his last 4 appearances for Slovakia 💪 @sfzofficial | #EURO2020 https://t.co/6GlcF6BIiu
— UEFA EURO 2020 (@EURO2020) 1623712165000
Too reliant on their top-scorer Robert Lewandowski who made no impact on the game, Poland failed to spark as coach Paulo Sousa’s unfamiliar 4-3-2-1 formation struggled against a compact Slovakian outfit.
Lewandowski’s big-stage goal drought continued as he has now scored once in his last 11 European Championship and World Cup games with Poland stretching their poor record in opening matches at major tournaments to one win in their last 10.
Slovakia, rated as rank outsiders in a group including former world and European champions Spain and Sweden, celebrated their fourth win in the last five games against the Poles with a gritty performance.
Slovakia coach Stefan Tarkovic heaped praise on Skriniar who has scored three goals in 41 games for Slovakia.
“Skriniar put in a great performance today but I think the whole team played very well,” Tarkovic told Slovakian television.
🇸🇰🇪🇸🇸🇪🇵🇱 𝐆𝐫𝐨𝐮𝐩 𝐄: Who are you backing to go through?#EURO2020
— UEFA EURO 2020 (@EURO2020) 1623707861000
“We prepared for Poland’s attack and Lewandowski was a focal point of course.
“It was a battle between Skriniar and Lewandowski, but I am really happy that other players like (Peter) Pekarik and (Lubomir) Satka also managed to neutralise a great player like Lewandowski.”
Lewandowski acknowledged Poland were below par and needed to improve ahead of their next match with group favourites Spain in Seville on Saturday.
“Such mistakes as with the second goal are unforgivable at the Euros,” he told Polish television.
“If we eliminate today’s mistakes we hope to play better against Spain but we are aware that they are a stronger rival. We need to take the responsibility ourselves.”
Poland were on the front foot in the opening stages but fell behind as Robert Mak beat two defenders on the left flank with neat footwork and cut inside before his shot hit the post and rebounded off Szczesny into the net.
🇸🇰 Match-winner & warrior Milan Škriniar takes the plaudits 🥇@Heineken | #EUROSOTM | #EURO2020 https://t.co/PpKVB0baZZ
— UEFA EURO 2020 (@EURO2020) 1623694149000
Slovakia continued to soak up pressure but always looked dangerous on the break and their central midfielder Juraj Kucka curled a long-range effort over the bar in the 27th minute.
Krychowiak came up with Poland’s first meaningful effort when his shot from 25 metres sailed over, with Slovakia’s compact defence cutting out the supply routes to striker Robert Lewandowski.
Poland’s top scorer Lewandowski failed to make any impact in the first half and scuffed an attempt from inside the penalty area woefully wide in the 42nd minute.
Poland levelled 30 seconds after the break as Mateusz Klich released Maciej Rybus and the left back squared the ball back for Linetty to scramble a close-range shot into the far corner.
The Poles were left with 10 men after Krychowiak received his marching orders for a clumsy challenge on an opponent and Skriniar, who man-marked Lewandowski out of the game, struck with some aplomb at the other end.
The Poles failed to clear a corner and Skriniar, who won the Serie A league title with Inter Milan this season, gave Szczesny no chance with his fierce low shot from 14 metres.
Lewandowski glanced a close-range header wide in the 87th minute as Poland pressed in the closing stages and Jan Bednarek missed their final chance in stoppage time when he fired wide from inside the penalty area.
Slovakia play their next match against Sweden in St Petersburg on Friday.