Carlo Ancelotti's men are unbeaten in La Liga since October and are now three points clear at the top of the table after months of grappling for the lead with Barcelona and Atletico Madrid.
The Italian said earlier in the season that Real's defensive record was their main issue, and Mendieta believes he has been proved right as Los Blancos have drastically turned around their defensive fortunes.
The Bernabeu side have kept nine clean sheets in their last 14 games after they could only manage two in the previous 13.
And the former Valencia midfielder says the key is their improved concentration as they have patched up a defence prone to shipping goals early on.
"They don't concede many goals in the first 20 minutes, like they used to," said Mendieta.
"They don't concede goals straight after they score, so that's given them more confidence and is making them more solid at the back."
With progression to the quarter-finals of the Champions League all but guaranteed after their 6-1 win in Schalke in the first leg, and a Copa del Rey final to go along with their advantage in the league title race, silverware is a distinct possibility at the Bernabeu after a trophy-less season last term.
Mendieta hailed the impact of full-back Daniel Carvajal in the current campaign, but believes Real's improved record at the back is not just down to their depth of defensive talent, but also the hard work put in by the midfield.
"[Ancelotti]'s found the back four, with Pepe and [Sergio] Ramos in good form," said Mendieta.
"The full-backs are as well, with Marcelo and [Daniel] Carvajal who, for me, is one of the revelations of this Real Madrid side - but it's also the midfield.
"Angel Di Maria works so hard, both in attack and defence. Xabi Alonso and [Luka] Modric as well.
"I think Modric is giving something else to that midfield that [Sami] Khedira didn't do, and that's made Real Madrid solid at the back."