Not to be too pedantic, but that's only correct if whatever it is weighs one kilogram per liter, which for example is only accurate for water at a 4 degrees C (It's less dense above and below)
You cannot directly convert from mg/liter to parts per million that simply without knowing the atomic weights of what you're measuring and doing some math. But mg/L=ppm is usually not going to throw your measurements off any more than the hobby grade equipment and test kits do.