In real life, it was an unhappy ending, alas.
I was looking for a photo of the doctor whose story was made into "Lost Boundaries". I found one and he doesn't look like Mel Ferrer. He kind of looks like the wife's father in the movie.
Apparently, in contrast to the film's happy ending, there wasn't such a nice tolerant ending for the actual doctor in New Hampshire. He was fired from his hospital post in 1953. (I assume that the picture of him as a G.P. was poetic license in the film. In this article and in his obituaries, he is listed as a radiologist.) This article was on a flickr page: http://www.flickr.com/photos/vieilles_annonces/1209636288/#/photos/vie illes_annonces/1209636288/lightbox/
In case it disappears, here's what it says:
"'Lost Boundaries' Doctor Ousted, Charges Bias
Dr. Albert C. Johnston, Negro physician in Keene, N. H., whose story of passing for white was told in the movie Lost Boundaries, was fired from his post as radiologist at Keene's Elliott Community Hospital. Chester Kingsbury, hospital board president, said racial prejudice was not the reason for the dismissal, cliaimed that Dr. Johnston could not devote full time to the job. Dr. Johnston said he would not seek reinstatement, explained there was "no doubt whatsoever" that he was fired because of the film of his life. "They have been picking on me ever since my story came out (in 1949). I don't give a darn for the job itself, but I'm concerned over the fact that I was fired because I'm a Negro," he declared. the physician said he learned that the hospital was looking for a ne radiologist soon after he let his children know their racial identity in 1947, added that "somebody began knifing me."