For me Zimmer gets the nod, hands down, for reasons exhaustively detailed upthread. All the contenders fall short of his special, historic level of incompetence, though in my mind Johnson ('75) and McNamara ('86) will always have much to answer for.
Let's not allow recency bias to blind us to the fact that our team had some idiot managers in the more distant past. The thread title specifies "in your lifetime," and I'm 82. So I was alive but not aware when Joe McCarthy forged a special record of stupidity in 1948.
By 1948 McCarthy the one-time genius "push-button manager" of the Cubs and then the Yankees in their Second Dynasty in the 1930's, was a faded shell of what he'd been in his days of great success.
He subscribed to a lot of notions and superstitions, groundless and unique to him, that were laughable to his players, though such was his stature, still, that nobody would have dared challenge him. Everybody smoked in those days, and one of McCarthy's quirky beliefs was that any man who smoked a pipe was a sedate pussy who could never be a manly, competitive baseball player. The story is told that one of his players, smoking a pipe, was told that McCarthy was approaching and in terror stuck the lighted pipe in his pocket, where it burned a hole in his trousers.
More important, probably, was the fact that he had become a full-blown alcoholic; not just a heavy drinker but a sodden lush.
How, apart from these factors -- age-based decline and alcoholism -- are we to account for the fact that in a one-game playoff with Cleveland for the AL pennant in 1948 McCarthy gave the starting nod to the pedestrian Denny Galehose...with both Mel Parnell and Ellis Kinder rested and anxious to take the mound?
The result was predictable, and I can still remember how upset my dad was, though I didn't quite understand why. Cleveland took the flag and went on to beat the Braves in the Series, and Cleveland hasn't won it since.
So, fellow SOSH'ers, when we're listing execrable Red Sox managers, let's not forget Marse Joe.