In 1912, Mathewson gave another stellar performance. Capturing the pennant, the Giants were fueled by the stolen-base game and a superior pitching staff capped by Rube Marquard, the "11,000-dollar lemon" who turned around to win 26 games, 19 of them consecutively. In the 1912 World Series, the Giants faced the Boston Red Sox, the 1904 American League pennant winners who were to face the Giants in the World Series that year had it not been cancelled. Though Mathewson threw three complete games and maintained an ERA below 1.00, numerous errors by the Giants, including a lazy popup dropped by Fred Snodgrass in game seven, cost them the championship. The Giants also lost the 1913 World Series, a 101-win season cemented by Mathewson's final brilliant season on the mound: a league-leading 2.06 ERA in over 300 innings pitched complemented by 0.6 bases on balls per nine innings pitched.
Mathewson garnered respect throughout the baseball world as a pitcher of great sportsmanship. He was often asked to write columns concerning upcoming games. In 1912, Mathewson published his classic memoir Pitching in a Pinch, or Pitching from the Inside, which was admired by poet Marianne Moore and is still in print. Years later, Mathewson co-wrote a mildly successful play called The Girl and The Pennant. He went on to pursue more literary endeavors ending in 1917 with a children's book called Second Base Sloan. One of the journalists to unmask the 1919 Black Sox, Hugh Fullerton, consulted Mathewson for information about baseball gambling. He trusted Mathewson for his writing intellect, as well as his unbiased standpoint. Representing the only former ballplayer among the group of investigating journalists, Mathewson played a small role in Fullerton's exposure of the 1919 World Series scandal.