Noun. Mom. “Some of the patter is awkward, but [Liza Minelli] is great about both her mum and dad.” (Vincent Canby, New York Times, December 19, 1999) “Actress Barbra Streisand is set to play Seth Rogen’s mum in new movie My Mother’s Curse.”(Monsters & Critics blog, January 31, 2011) Google Ngram.
notoneoffbritishisms.com 
This may result from different transcriptions of similar-sounding “baby-talk”. “Mom” in American English, and “Mum” in British English, both suggest an open back vowel realization. “Mom” in British English would suggest a much more rounded pronunciation.
Everyone on both sides of my extended New England family calls our mothers Mum or Mumma. It never seemed a Britishism, but rather a New England-ism. People in Nova Scotia, and quite likely most of Canada, also use this.
@dw This is true. They’re pronounced pretty much the same, just spelled differently in order to get the same sound in different accents.
“Mum” is borderline, but any North American child who says “Mummy” should be smacked.
I grew up and lived on the southshore of Boston for 15 years and I’ve always said ‘mum’ and ‘mummy’. Never did I think of it as a Britishism ’till later, but even then I’ve felt it was a New England thing that stuck around along with a few other terms like ‘rubbish’ instead of ‘garbage’. All of my friends said it too.
Here in Britain, in the 50s, my parents wouldn’t have stood for ‘Mum’ and ‘Dad’ (too common!), so Mummy and Daddy it was.
A class thing…..