Was Lamech the first polygamist (Gen 4:19)?

Perhaps: he was the first one recorded. That this was frowned upon even in OT days is arguably shown in Gen 2:24: “Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.” Other texts show that the better way, at least, was to have only one wife: “Neither shall he multiply wives to himself” (Deut 17:17; one of the rules for the future kings of Israel). Similarly, “A bishop then must be blameless, the husband of one wife” (1 Tim. 3:2). This meant that in those times, as with slavery, polygamy was tolerated by God, probably in a concession to the inevitable practices of primitive man.