Just created a USB Linux Mint with persistent
You forgot to indicate how you created your flash. If you used the Universal USB Installer (Pendrivelinux), it indicates on the main page that persistence won't work unless it is formatted FAT or FAT32. If you used something else, it would be a good idea to mention what.
Create a file in the /home/user directory, reboot to see if it is there. I've never used an ntfs formatted flash drive so don't know if it would work for Linux.