Actually, the 1981 Kansas City Royals had the worst winning percentage for a team that still made the playoffs. In a strike-shortened season, they finished with a 50-53 overall record, which was good enough for 4th place in the AL West (overall). However, MLB decided to have a Divisional Series that season (due to the strike) and the winners of the first and second halves of the season, for each league (in their respective divisions), would play one another before determining a league champion. That year Kansas City won the second half, posting a 30-23 record but still finished 50-53 overall with a .485 winning percentage. It's worth noting that they were swept 3-0 in the 1981 ALDS by the Oakland Athletics.
Through the 2008 season, the Tampa Bay Rays have the worst lifetime winning percentage of any MLB team with .417 (742-1037).
50%
Rangers, twins
In MLB as of the start of the 2009 season, the lowest regular season winning percentages are: 1) Tampa Bay Rays: 742-1037 .417 2) San Diego Padres: 2933-3421 .462 3) Texas Rangers: 3570-4059 .468
Chicago Cubs
In MLB in the 2009 season, in the National League the average on base percentage was .331 and the average slugging percentage was .409. In the American League, the average on base percentage was .336 and the average slugging percentage was .428.
Through the 2008 season, the Tampa Bay Rays have the worst lifetime winning percentage of any MLB team with .417 (742-1037).
Johan Santana
50%
The team with the worst winning percentage in the history of MLB -- the 1899 Cleveland Spiders -- still managed to win 20 games out of the 154 they played that year. This dismal .130 winning percentage is so bad that it is extremely unlikely that any MLB team will do worse. The 1916 Philadelphia Athletics -- the third worst team in MLB history -- had a .235 winning percentage, 100 points better than the Spiders.
The Washington Nationals
In MLB, 1.22 by the 1876 St. Louis Brown Stockings of the National League.
Seattle Mariners
Giants
Rangers, twins
211
In MLB as of the start of the 2009 season, the lowest regular season winning percentages are: 1) Tampa Bay Rays: 742-1037 .417 2) San Diego Padres: 2933-3421 .462 3) Texas Rangers: 3570-4059 .468