The film, the streaming service's first cinema-quality project, made only $50,699 (£32,800) in the 31 US cinemas on which it was shown on release.
To date the film has earned gross box office takings of $83,861 (£55,000).
But Sarandos told Deadline the film had overall had "a bigger audience than any specialty film could ever hope".
Netflix released the film, to which it bought the distribution rights for $12m (£7.8m), simultaneously on its own platform and in a limited number of cinemas in the US on 16 October.
The film, directed by Cary Fukanaga and starring Idris Elba, focuses on child warfare in Africa.
It is understood it was given a limited theatrical release to qualify it for the upcoming awards season.
It is rare for Netflix to publicly reveal viewing figures, which means Sarandos' candour could be a sign of the service's commitment to this new venture.
Sarandos told Deadline Beasts of No Nation was, in the first week of release, the most watched movie on Netflix in all countries in which the service operates.
"We are just thrilled with the total audience reach of this film, not just in North America, but the world," he told Deadline.
"This was number one in really diverse places in the world - Japan, Brazil, Mexico, places where these films typically never even open.
"It's been incredibly gratifying to see these audiences respond to this film."
Netflix has 69 million monthly subscribers, 43 million of which are in the US, in more than 50 countries around the world.
Sarandos said there were no plans to increase the number of screens showing Beasts of No Nation in the US.
However, he said Netflix was planning to release the film theatrically in Africa, in Ghana and Nigeria.
"Netflix is not there, and obviously the film is of local importance, so we're doing that as well," he said.
Beasts of No Nation was released in the UK on ten screens to make it eligible for the British film award season.
Netflix's Ted Sarandos has said he is "thrilled" by the success of Beasts of No Nation, saying it had been viewed by more than three million US subscribers.