The forward’s form was questioned before kick-off but he headed in Dani Carvajal’s cross to open the scoring.
He also created the second goal which arrived moments after Schalke’s Felix Platte had smashed against the bar.
Marcelo capitalised, driving into the top corner, and the holders will take a healthy lead to Madrid on 10 March.
Los Blancos ended a 12-year wait to capture ‘La Decima’ – a 10th European Cup win – last season and thumped Schalke 9-2 on aggregate at the same stage of the competition.
While this result may not lead to such a convincing margin of victory, it was achieved with dominance from the start and Carlo Ancelotti’s side ended with 61% possession.
Los Blancos have now won 10 straight games in the competition and with James Rodriguez to return from injury to assist Gareth Bale, Benzema and Ronaldo, they have a stockpile of talent to threaten retaining the trophy.
Ronaldo has a staggering 58 goals in 58 appearances in the competition but judged on his own incredible standards, the Fifa World Player of the Year’s form was a hot topic at Ancelotti’s pre-match news conference.
It took just 26 minutes for him to repay his manager’s backing, beating Schalke goalkeeper Timon Wellenreuther to the ball to head home.
It was his 12th goal in as many away games in the competition and Madrid should have taken further control minutes later but Benzema could only drive at the goalkeeper’s legs when clean through.
Schalke’s solid early start – built on deep defending when Madrid kept possession – had been undone and only Wellenreuther’s diving save prevented Ronaldo a second with a dipping free-kick.
Roberto Di Matteo’s side lost ex-Madrid striker Klaas-Jan Huntelaar to injury before the break but stuck to their task defensively in the second period and will rue Platte’s narrow miss.
The substitute rattled the woodwork from 18 yards with Iker Casillas beaten and Atsuto Uchida’s follow up lacked the power or direction to level.
It would prove a key moment as when Ronaldo dribbled from the right flank to feed Marcelo, the Brazilian full-back drew his right foot to thunder into the top corner.
Only Barcelona and Juventus have won by a two-goal margin in the Champions League at the Bernabeu in the last 10 years but Schalke will need to do just that if they are to reach the quarter-finals for only the third time in their history.
Source: BBC Sport