A novel maize variety designated PH1M5G and seed, plants and plant parts thereof. Methods for producing a maize plant that comprise crossing maize variety PH1M5G with another maize plant. Methods for producing a maize plant containing in its genetic material one or more traits introgressed into PH1M5G through backcross conversion and/or transformation, and to the maize seed, plant and plant part produced thereby. Hybrid maize seed, plant or plant part produced by crossing the variety PH1M5G or a locus conversion of PH1M5G with another maize variety.