The Dominican smashed three goals and Enzo Zidane weighed in with his first for the Merengue as they stormed into the next round of the domestic competition
Zinedine Zidane's son Enzo scored on his Real Madrid debut and Mariano netted a hat-trick as the European champions prepared for El Clasico with a 6-1 win over Cultural Leonesa in the Copa del Rey.
Having won the away leg of this last-32 tie 7-1 a month ago, and with a trip to face Barcelona at Camp Nou just three days away, Wednesday's meeting against third-tier opposition at the Santiago Bernabeu was little more than a warm-up for Madrid as they sealed a 13-2 aggregate success.
Zidane made eight changes from last weekend's 2-1 win over Sporting Gijon, with the likes of Cristiano Ronaldo, Karim Benzema and Luka Modric all rested.
In their absence, striker Mariano - making his maiden start - took his chance to impress by scoring the first of his goals after just 23 seconds, his first-half double coming either side of James Rodriguez's header.
1 - Mariano Díaz has scored the fastest goal for Real Madrid under Zinedine Zidane (23 seconds - all competitions). Electric.— OptaJose (@OptaJose) November 30, 2016
It was the visitors who scored the pick of the goals thanks to Yeray Gonzalez's fine long-range effort just prior to the break, before Mariano teed up half-time substitute Enzo for his opening goal in Madrid colours, scoring with his very first shot.
Mariano completed his hat-trick with a downward header three minutes from the end and, as Cultural tired, Cesar Morgado put through his own net for the sixth.
The tie was over as a contest by that point and all eyes will now turn to Saturday's Clasico showdown, where LaLiga leaders Madrid will recall their superstars in a bid to extend their advantage over Barca to nine points.
This was a lost cause for the visitors from the off and Cultural got off to a nightmare start when they fell behind almost immediately.
Ivan Gonzalez received a pass from his goalkeeper on the right-hand side of the box but his touch let him down, allowing Marco Asensio to nip in and cut the ball back for Mariano, who tapped home.
Madrid doubled their lead midway through the first half as James powered home a header from Dani Carvajal's excellent right-wing cross.
Carvajal was the provider again shortly before half-time, finding Mariano with a clever pass around the side of the Cultural defence and the forward made no mistake in taking a touch and rifling a volley into the roof of the net.
Mariano was upstaged on the stroke of the interval, though, as Yeray Gonzalez thumped a 25-yard effort into the top-left corner to give the smattering of travelling Cultural fans something to cheer as the teams headed down the tunnel.
Enzo entered the action at the start of the second period and made his mark just after the hour, shooting low into the left-hand corner from Mariano's pass.
Alvaro Tejero's cross was gleefully buried by Mariano for his third, Cultural's misery compounded by the unfortunate Morgado as Madrid made it 32 matches without defeat in all competitions heading into El Clasico.