I've been corresponding with a British friend/colleague in the sciences (I'm in the humanities) about British job applications, and noticed that he lists his master's degree distinction on his CV. Since to my knowledge we in the US don't have distinctions or honors for master's degrees, and I have a 4.0 GPA for my master's degree, should I note 'distinction' too? Or is this unnecessary/misleading?
You do not have a Masters 'with distinction' so you would not be stating the truth if you said you did (the UK degree certificate will state 'with distinction'). You could note the GPA in your CV because that would not be misleading.