Madrid took control of the match after a lackluster first half in which Valencia had the most significant scoring chances and a goal disallowed by video review.
Madrid started strongly and could have gone ahead after 12 minutes when Benzema threaded a low ball through to Eden Hazard, but the Belgian shot straight at Jasper Cillessen’s feet from just inside the area.
Moments later, Valencia had an even better chance as Maxi Gomez sent Rodrigo Moreno away and the Spain striker drilled a low shot against the post, with Thibaut Courtois getting a vital touch.
Valencia then thought they had taken the lead after 21 minutes when Rodrigo anticipated a Carlos Soler pass and popped up at the far post to beat Courtois from close range.
There initially appeared to be nothing wrong with the move, but VAR had a look at the position and involvement of Maxi Gomez, who was a fraction offside and was adjudged to have been interfering with play by the referee as he was in front of Raphael Varane as the ball was played through.
After the official went over to watch the replay at the side of the pitch, the strike was ruled out and Valencia felt hard done by, with Jose Luis Gaya booked for his protests.
It was a lucky escape for Madrid and for Sergio Ramos, who had been out of position.
Zinedine Zidane’s side had plenty of the ball in the remainder of the half, but their most dangerous moments came from Carvajal crosses and Benzema’s understanding with Hazard was not as good as against Eibar on Saturday.
The second half followed a similar pattern, but Madrid finally found the breakthrough just after the hour as Hazard found space on the left, cut a ball back to Modric, ran into the centre for the return, and squared for Benzema, who slid a low shot past Cillessen.
The Frenchman sought to return the favour in Madrid’s next attack, threading a low pass through to Hazard, but the former Chelsea attacker fired high and wide with only Cillessen to beat.
After Valencia had made four of their permitted five substitutions, Zidane then sent on Asensio for his first appearance since he ruptured his anterior cruciate ligament last July and the Mallorca-born midfielder scored with his very first touch.
Mendy got to the byline on the left and sent in a cross for the Spanish international, who beat Cillessen with a first-time shot with his left foot to put Real Madrid 2-0 in front.
Benzema then put the icing on the cake with a wonderful strike near the end as he controlled with his right foot in the area and volleyed a sweet strike into the top corner with his left after 87 minutes.
Valencia substitute Kang-in Lee was sent off for hacking Ramos with a minute left and it was en evening to forget in the end for the visitors.
Madrid moves back to within two points of leaders Barcelona with nine rounds remaining, while Valencia is down in eighth, six off the Champions League positions.