Please give us ideas to lock front door so that mom doesn't walk out.

Child proof knob covers.

But, it sounds like mom is at the stage she should not be left alone. Find help so you can get out when you need to. Or find appropriate level of care for mom.

You’ve had a long road with your mom. It sounds like she now needs to be watched all the time. Like the others, I worry that she is now like a toddler and her dementia is progressing beyond the “early” stages. You would not leave a toddler home alone. But, you’re right. You cannot take her everywhere with you. Research the availability of in home care for her. Someone experienced with people with her issues. Or, maybe adult daycare. If she becomes anxious when she doesn’t see you, maybe a low dose of anti-anxiety meds just before you go out.

I can't imagine the hardship this will impose, but I agree with those here who have said Mom likely isn't safe to leave alone any more. If you put in something Mom cannot easily access, then she would be trapped in the event of a fire. She could, if she has access to the kitchen when you are gone, be in danger of actually setting a fire, as well.
So sorry to hear that it has reached this stage, but I can't see an answer for you that would be safe for her.

Sorry, she’s not safe to be left alone anymore. She needs a caretaker or a place where she’ll be watched all the time

You are looking for a solution to prevent her from getting out of the house while you run errands is how I read your post. Are you not concerned about a potential fire in the house...how would she get out?

Instead of staying alone, you might consider adult day care, so she gets supervision and you can go out without worrying.

For everyone worried that someone could not escape a fire, most dementia patients would not even realize they were in danger because of the fire and not even try to escape. They would be overcome by the smoke and become unconscious.

Keeping them safe from the real danger of walking off is more important than what ifs.

Look at your local hardware store for child locks, place them high up and mom will never even see them. My granny was was kept in with little tab locks that fold over the door frame to stop the door from opening. Pushing them up opened them. It really is important to put anything, even a chain lock above eye level. Demented people don't look up.

It is scary and tiring to be chasing a wanderer all the time. Keeping them securely inside does allow you a level of comfort to get household chores done.

It is probably time for mom to have 24/7 supervision, I am sorry. That requirement changes everything and usually leads to facility care, you can't keep up with the energy they have, it takes a village. My granny would pace the halls in her wheelchair chair nonstop for 18 hours a day.

I'm sorry your mom's dementia has declined to this point. As others have mentioned I question her ability to be alone......If she tries to get out, what else is she doing to put her in harm's way? I struggle with my own mom's limitations, but she insists on staying home and dad gives in. If I were making the decisions she would have been in AL yesterday. Dementia is highly individual....your mom could be progressing at a faster rate. For her safety and your peace of mind find other alternatives.....at least a part time caregiver while you are out. Hope this helps.

A friend installed deadbolts backwards. So from the outside you just twist the lever. In case of emergency. But inside you need a key to leave.
Hope this helps -
Sparkles

For the door, I used a sliding latch placed at the top of the door to keep rambunctious children inside the house and that would probably work for someone with dementia too. You might consider placing a latch on the outside of the door too so you can engage the latch as you leave and anyone on the outside could still open the door as needed. My mother has MCI with mobility problems and when I occasionally leave her for 30-45 minutes while I pickup some food or a grand-kid, I ask a neighbor to keep an eye on the house and let herself in if the security system alarm goes off. The security system includes smoke alarms and a panic button.

I see in your profile you would like to move Mom into a care facility after you get the financial side organized. Please consider contacting the Area Agency for Aging (AAA) social worker to see if there is an adult day care where your mother could stay a few hours daily during the week and to see if Mom would qualify for some in home care hours. AAA could also help you get the financial stuff in order for Mom's placement. Good Luck.