What can I do when mom with dementia has lost control of her finances? I do not have power of attorney.

I would definitely seek legal advice as to whether you could get power of attorney at this point. It is a much simpler means of handling someone's affairs than having to get guardianship. I have guardianship of a brother and sister and POA for my mother. I must prepare an audit of every penny I spend as guardian for my siblings and must pay filing fees and pay to be bonded to handle their monies. Being POA for my mother has been much less tedious and less costly. Check all avenues out in great detail before pursuing.

ask an elder care attorney for a consultation - I found a good one near me by just searching on Google, and chose based on the web site having information about what we needed to address most...God bless, this is not easy stuff but has to be done.

File for guardianship if you her doctor or a doctor will say that she is not competent to handle her own business in a business like manner.