Breaking in as a 27-year-old rookie, Jansen became a key member of the
New York Giants starting rotation from 1947 to 1953, twice winning more than 20 games. He was purchased from the
Triple-A Seals after leading the Pacific Coast League in wins (30),
earned run average (1.57) and
winning percentage (.833) in 1946. In his rookie major league season in , Jansen won 21 of 26 decisions, leading the
National League in winning percentage (.808), and finished second in the voting for
Rookie of the Year behind the
Brooklyn Dodgers'
Jackie Robinson. He pitched five innings of one-hit scoreless baseball in the
1950 All-Star Game, which lasted 14 innings. In , he paced the NL-champion Giants with 23 victories and helped lead their improbable August and September comeback against the Dodgers. Jansen was the winning pitcher in the famous game on October 3, 1951, featuring the
Shot Heard 'Round the World. Jansen, however, lost his only two decisions in the
1951 World Series. Jansen also won 19 games (1950) and 18 games (1948) for the New York club. As an indication of the low salaries of even accomplished players in the mid-twentieth century, Jansen worked in a hardware store in Forest Grove, Oregon, during the off-seasons of his best years. Arm miseries kept Jansen from a major role in the Giants'
world championship; he spent part of that season inactive, as a coach. His playing career ended after eight appearances with the 1956
Cincinnati Redlegs. During his nine-year NL career, Jansen won 122 games and lost 89 (.578) with an ERA of 3.58. He had five 15-win seasons and two 20-win seasons. ==Coaching career==