The best of them all was actually a very famous squad: the 1951 Giants, who memorably won the pennant on Bobby Thomson‘s home run to end a three game playoff against the Dodgers. The Giants, who went 95-59 (.624) on the year, had their slump at the very beginning of the year. After a 2-1 start, they flopped to 2-11. They won nearly two-thirds of their games the rest of the way.
Only one other .600+ club ever lost ten straight: the 1896 Reds. It’s OK – they never heard of you, either. They were in first place by a half-game with a record of 69-31 heading into August 20. By the time they posted win #70, they were 6.5 games back, having dropped 11 straight. They never fully righted themselves, and in the three weeks after the losing streak they fell to third, 12.5 games back. I have no idea what happened to them, but they just completely collapsed at the end.
Next worst were the 1925 A’s, who went 88-64. When they began their skid in late August, they were just one game out of first. After dropping 12 straight, they were nine game back, and they ended up losing the pennant by eight games to the Washington Senators. Then again, four of the dozen loses came at the hands of the Senators.
The fourth worst streak came from the 1961 Dodgers. Like the Reds, they were in first place in August when they began their slump. Their 10-game slide pushed them from 2.5 up to 3.5 down. They ended up losing the pennant by four games. They ended up 89-65 on the year.
While some of the above teams lost their pennant hopes late in the season, no team can claim a more dramatically timed losing streak than the fifth best team to ever lose 10 straight: the 1964 Phillies. They famously possessed a 6.5 game lead with only 12 to play, but by dropping ten in a row they eliminated themselves from the postseason before winning their next game. They finished with a 92-70 record.
Only two other squads with 10-game slumps finished with winning records over .550: the 1987 Brewers (91-71) and 1932 Pirates (86-68). The Brewers were famous for winning their first 13 straight games. They were 20-3 before a slide dropped them to 20-15. Now that’s a streaky team.
Like many teams here, the Pirates were in first place when they began their slide. Unlike all the others, they were also in first when the skid ended. After losing their tenth straight on August (another August slump!), they were 0.5 ahead of second with a 59-48 record. As you might guess, it was a very tightly packed league. Heck, they were only 8.5 games out of seventh place. Thus when they dropped 12 of their next 17, they fell out of the running. Even a ten-game winning streak that took place immediately after that spell couldn’t lift them back into the driver’s seat.