Mijnheer and meneer have the same meaning, but mijnheer is very old fashioned. It's the old pronunciation. Anyway, the Netherlands is very informal and we do not often use this word. You can use it in a customer to service person conversation. Maybe in a pupil teacher situation. But even that is not necessary. We called our teachers by their names at my school.

Meneer en mijnheer betekenen hetzelfde. Mijnheer is ouderwets, maar wordt soms nog in schrijftaal gebruikt (brieven ed).