However, in first-half stoppage time Saudi were awarded a second penalty - after a VAR review - when Ali Gabr was deemed to have pulled down Al Muwallad in the area.
He justified his inclusion in the 41st minute, flying to his right to push Al Muwallad's powerful penalty onto the crossbar after Yasser Al-Shahrani's cross had hit Ahmed Fathy's arm.
Egypt, who fielded Essam El Hadary, the oldest ever player at a World Cup aged 45, are without a win in their last six appearances at the World Cup.
Saudi Arabia and Egypt have only pride to play for as they take to the 2018 World Cup stage for the final time.
The first match was a 1-0 defeat to Uruguay, whereas the second was a 3-1 loss to Russian Federation.
El-Hadary, who is regarded as one of Africa's best goalkeepers of all time, started his career in 1996 and is a four-time African Cup of Nations champion.
And it was first blood to Egypt as Salah recorded his second goal of the tournament.
Salah squandered an easier chance to score after breaking Saudi Arabia's questionable offside trap, lacking his earlier composure and clipping an effort wide of the right-hand post.
As the game entered the final quarter, Housain Al-Mogahwi was given the chance to score his first worldwide goal at the back post but the midfielder's header was tipped over the crossbar by El-Hadary.
Saudi Arabia winger Salem Al-Dawsari scored a last-gasp victor as Mohamed Salah's Egypt suffered a 2-1 loss in their Group A dead rubber in Volgograd on Monday.
Salah scored on Egypt's first real opportunity in the 21st minute. Saudi Arabia, was also eliminated after the second match.
Despite scoring from the penalty spot, Salah looked like he was still feeling the effects of his injured shoulder in that game.