Barbara Grant, the actor's fifth wife, attended Wednesday's screening of the film with Grant's "Father Goose" co-star, French actress Leslie Caron, BBC reported Thursday.
"Cary was the most extraordinary person I have ever met. He was a star but always remained down-to-earth. This is a wonderful film, which looks at the real Cary Grant," Barbara Grant said.
The couple married in 1981. His widow was 47 years younger than Grant, who would have turned 100 this year.
The film, directed by Robert Trachtenberg, is narrated by British actress Helen Mirren and features interviews with Grant's "North by Northwest" co-stars Eva Marie Saint and Martin Landau. It is set to be released later this year.
Grant made more than 70 films, including the classics "The Philadelphia Story," "Bringing Up Baby," "To Catch a Thief" and "An Affair to Remember."
Grant died in 1986 while touring with his one-man show "An Evening with Cary Grant."