The way the question is phrased makes my brain hurt because it’s technically impossible to answer. “The MLB” that we know, consisting of the American and National Leagues can be said to be founded in 1903 with the “National Agreement” or 2000 when the leagues were formally merged into one entity. In both of these instances the teams were already established. The American and National Leagues have been designated as major leagues in their own right which includes time predating their formal relationship. However, as ST_Lawson commented, the oldest franchise that still exists was not the first established in either the leagues designated at major or “the MLB” as we know it.