Mbappe opened the scoring from the penalty spot in the 19th minute before completing his brace within the first 30 minutes of the game.
Jude Bellingham increased the lead to 3-0 shortly before half-time with Alvaro Carreras sealing the win with a lovely finish into the top corner in the 82nd minute.
The scoreline was even a bit flattering for Valencia as Vinicius Jr had a penalty saved by Julen Agirrezabala.
With Barcelona set to play on Sunday, the victory increased Los Blancos' lead over the Catalans to eight points, with Barca dropping to third in the standings behind Villarreal, who also picked up a win on Saturday.
El Submarino Amarillo thrashed Rayo Vallecano 4-0 at Estadio de La Ceramica to move up to second in LaLiga after an impressive start to the season.
Gerard Moreno opened the scoring in the 22nd minute as Villarreal took a 1-0 lead into halftime.
They broke the game open in the second period with Alberto Moleiro doubling their advantage in the 56th minute before Santi Comesana made it 3-0 just two minutes later.
Ayoze Perez rounded out the scoring of the dominant performance with his strike in the 65th minute.
Atletico Madrid also produced a strong performance in the 3-0 win over Sevilla at Yiah Air Metropolitano.
After a tightly contested first half, action exploded in the second period after Julian Alvarez opened the scoring from the penalty spot in the 64th minute.
Thiago Almada made it 2-0 in the 77th minute with Antoine Griezmann completing the scoring in the 90th minute.
The tightest match of the day was at Reale Arena, where Real Sociedad overcame Athletic Club 3-2.
Brais Mendez opened the scoring in the 38th minute for the hosts before Gorka Guruzeta restored parity just four minutes later.
Goncalo Guedes restored Sociedad's lead shortly after half time before RobertNavarro got Athletic back on level terms again in the 79th minute.
However, the home side would have the last laugh as Jon Gorrotxategi bagged a late winner two minutes into stoppage time at the end of the game.
