Haefner's professional career did not begin until 1937, when he was 24. After five straight seasons of double-digit victories in minor league baseball, he was acquired by Washington in February 1943. As a rookie, he worked in 36 games, with 13 starts, and put up an 11–5 won–lost record and a 2.29 earned run average in 165.mw-parser-output .frac{white-space:nowrap}.mw-parser-output .frac .num,.mw-parser-output .frac .den{font-size:80%;line-height:0;vertical-align:super}.mw-parser-output .frac .den{vertical-align:sub}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}1⁄3 innings pitched. He was third in the Junior Circuit in ERA and fourth in winning percentage (.688), while the Senators ended up second in the league to the New York Yankees—albeit by a distant, 131⁄2-game margin. His workload increased in 1944, the first of three straight seasons in which he exceeded more than 225 innings pitched.
In 1945, knuckleball aces Haefner, Dutch Leonard, Johnny Niggeling and Roger Wolff started 111 of the Senators' 154 games and accounted for 60 of the club's 87 victories, as Washington contended for the American League title up until the closing day of the season, before finishing 11⁄2 lengths behind the Detroit Tigers. Haefner went 16–14 (3.47) with 19 complete games and three saves. In 1946, the first postwar season with many military veterans returning to major league service, Haefner continued his effective pitching, winning 14 games in 25 decisions and posting a 2.85 earned run average with 17 complete games. He won ten more games in 1947 before his performance declined to a poor 15–32 record over his final three MLB seasons. However, on May 10, 1949, against the defending world champion Indians at Cleveland Stadium, Haefner threw a complete game one-hitter for a 1–0 victory. Only Baseball Hall of Famer Larry Doby's first-inning single prevented Haefner from throwing a no-hitter. A little over two months later, on July 21, the Senators sold his contract to the White Sox.
In eight seasons and 261 career games pitched, Haefner compiled a 78–91 record with an earned run average of 3.50. He had 91 complete games, 13 shutouts and 13 saves. Haefner allowed 1,414 hits and 577 bases on balls, with 508 strikeouts, in 1,4662⁄3 MLB innings pitched.

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