When did political bias in the worldwide MSM exactly start?
I don't think it is bias it is communist propaganda.
Slowly they have gained total control.
Cronkite.
There's always been media bias but in past times it was mostly out in the open, not sneaky and posing as "objective" reporting.
Strong Communist influence in the media began at least as early as the 1920s. Read "Witness" by Whittaker Chambers for a glimpse of how it was. The current generation of vipers snuggled into secure, safe places during WW II and under cover of the Vietnam anti-war movement.
There have been repeated attempts to expose it, as in this fairly recent piece, but the public was apathetic and comfortable. Those who gave warning were considered "paranoid."