The Frenchman's 20th and 21st league goals - the second of which came from a contentious penalty - of the season ultimately earned a win at Estadio Alfredo Di Stefano that meant Barcelona's result against Osasuna was rendered immaterial.
Zinedine Zidane's men were in control against game opponents until Vicente Iborra's late header made for a nervy finish, but a 10th straight win since LaLiga's resumption means Madrid are champions with a game to spare.
Benzema has scored seven times in that run and it was fitting his contribution should round out their title success given the striker's goals and a solid defensive base have been the pillars of Madrid's campaign.
This game did not initially look like being the banana skin it had the potential to be and it felt a matter of time before Madrid found an opening, albeit bar Luka Modric's long-range drive their early endeavours proved fruitless.
The opener arrived after 29 minutes, though. Casemiro picked off Sofian Chakla's pass and Modric quickly laid into the path of Benzema, who drilled through the legs of Sergio Asenjo.
Asenjo denied Dani Carjaval early in the second half after the full-back showed quick feet to get a left-foot shot away.
Madrid were gifted a golden opportunity to double the lead when Sergio Ramos tore forward and was awarded a dubious penalty when the referee judged Chakla to have clipped Los Blancos' captain.
Ramos cheekily passed the ball off the spot for Benzema to score, only for the latter to have encroached, but Benzema clinically dispatched the re-take into the bottom-left corner.
Toni Kroos rattled the crossbar prior to Iborra's thumping header to set up a tense finale, in which Thibaut Courtois brilliantly saved from Bruno Soriano and Iborra in the same move.
Marco Asensio thought he had calmed the nerves in the final minute of added time only to be denied by the VAR, but it did little to dampen the mood of the new champions.
What does it mean? Madrid reign again in Spain
There are few who would argue Madrid are not worthy champions. The goals again have not flowed as freely as once they did in the halcyon days of Cristiano Ronaldo but Zidane's men have been far and away more consistent than deposed champions Barcelona.
Madrid have now won 11 trophies during Zidane's two spells in charge, while with 34 top-flight titles they have eight more than their Clasico rivals.
Benzema the Karim of Madrid's title-winning crop
Much of Madrid's success this term has been built on the back of a stoic defence that has been breached just 23 times.
But Benzema has been their chief goal threat and was once again outstanding at linking Madrid's midfield and attack. A landmark goal is just reward for a tremendous campaign and he has now registered 20-plus goals in consecutive LaLiga seasons for the first time.
Los Blancos accept Chakla gift
Chakla had already been booked having been caught out by Eden Hazard and his lack of awareness was pounced upon by Casemiro. He was unlucky with the penalty award but it was not a night the full-back will remember with fondness.
Key Opta Facts:
- Real Madrid have won their 34th LaLiga title - this is the most of any team in the competition's history.
- No other player has scored more in LaLiga since the restart in June than Karim Benzema.
- Madrid have just conceded 23 goals after 37 games in LaLiga this season, their best defensive total at this stage in the competition's history.
- Luka Modric has provided seven assists in LaLiga this season, his best ever return in a league campaign (LaLiga and Premier League).
- Benzema has scored all of his 10 penalties he has taken in LaLiga for Real Madrid - no current LaLiga player has netted more with a 100 per cent success rate.