You can't express mass in volts. A volt is energy per unit of charge. To get energy, you need to multiply by a charge.
One Joule of energy is what you get when you move one Coulomb of charge across a 1V potential.
One electronVolt (eV) is the energy you get from moving one electron's worth of charge across 1 volt of potential.
It's an accident of what we chose to be a Joule of energy and what we chose to be a Coulomb of charge, so there should be no expectation that this would turn out to be the mass of an electron (when divided by the square of the speed of light, which is unstated because everyone knows E = mc^2).
One Joule of energy is what you get when you move one Coulomb of charge across a 1V potential.
One electronVolt (eV) is the energy you get from moving one electron's worth of charge across 1 volt of potential.
It's an accident of what we chose to be a Joule of energy and what we chose to be a Coulomb of charge, so there should be no expectation that this would turn out to be the mass of an electron (when divided by the square of the speed of light, which is unstated because everyone knows E = mc^2).