Is Hayley Mills suppose to be American in this??!!

People didn't used to be bothered by such things in the early 1960's. Only in recent years have people begun to want everything to be perfect in movies.

I just chalked this up to her character (Nancy Carey) being from Boston. If you remember "The Parent Trap", Hayley also has her English accent (to the fullest) and the character (Sharon McKendrick) is from Boston. When she is portraying her twin sister Susan Evers, who is from California, she makes a great attempt losing the accent, nevertheless, it does poke through at some points.

As noted above, the accent was present to some degree in all her films and her performance was always delightful enough that no one seemed to mind. My personal favorite is from the song "Let's Get Together" in "Parent Trap":

Nothing could be greater
Say, Hey! Alligay-tah!

But strictly speaking, you're right, she's definitely in line for the Ashley Wilkes Award for Best Unexplained British Accent in an American (or quasi-) character.

I just watched this movie for the first time ever. Didnt see it as a kid that I recall but right at the very beginning when I realized that the red head boy was her brother and he didnt have an accent like her and then the little boy also I thought how can that be. Why would she have an english accent when her entire family doesnt. So it is kind of hysterical. At least years later in the movie Grease when they added Olivia Newton-John as Sandy and she told them she could not do an American accent they added the bit at the beginning where she tells Danny after the summer she has to go back home to Australia to let us know why she had that accent.

No one cared.

However, the family was quite wealthy before the father died. It's possible Nancy went to an upper class school and was taught to speak that way. Back then they had elocution lessons even in many public schools; you were taught how to speak in public via poetry and famous speeches learned in class.

You think Hayley in "Summer Magic" is bad/out-of-place?

Try Julie Andrews in "Thoroughly Modern Millie" - where she's supposed to be from the Midwest!

Really illogical - even stupid - circumstances...yet I LOVE both these movies.

Oh Hollywood - the "Land of Make-Believe"....

"Don't call me 'honey', mac."
"Don't call me 'mac'... HONEY!"

This was a Walt Disney 1963 kid's movie. Nobody cared. We were all mesmerized by Haley, the soundtrack, Eddie Hodges, Dorothy McGuire, Burle Ives, the beautiful cinematography, etc. I still find the movie charming today, accent or no accent.

Kind of like Arnold Schwarzenegger movies and his accent. Some things you just don't question.