In a 17-year career, Welch compiled a 211–146 record with 1,969 strikeouts and a 3.47 ERA in 3,092 innings. His 137 wins during the 1980s was third among major league pitchers during that decade, following Jack Morris and Dave Stieb.
Striking out Reggie Jackson in the 1978 World Series
Welch was the third starting pitcher in the rotation for the 1989 World Series champion Oakland A's, compiling a regular-season record of 17-8 and recording a win in his only start in the American League Championship Series against the Toronto Blue Jays. In an odd twist of fate, however, Welch did not throw a single pitch against the San Francisco Giants during the World Series itself. Just minutes before Welch was to take the mound in Game 3, Candlestick Park and the Bay Area were struck by the Loma Prieta earthquake, which caused extensive damage in the region and forced the postponement of the game. When the Series was resumed 11 days later, A's manager Tony LaRussa opted to re-use his Game 1 starter, Dave Stewart, for Game 3 in place of Welch, and his Game 2 starter, Mike Moore, for Game 4 in place of originally scheduled starter Storm Davis. The strategy worked, as the A's swept the Series in four games. For obvious reasons, the 1989 World Series was also known as the "Earthquake Series".
A two-time All-Star (1980 and 1990), Welch won 14 or more games in eight years, with a career-high 27 in 1990. He received the Cy Young Award that season, and was considered in the MVP vote. His 27 wins were the most by any pitcher since Steve Carlton also won 27 in 1972, and currently stands as the last time a pitcher has won 25 or more games in a season (the closest anyone has come to that mark since is 24, accomplished by John Smoltz in 1996, Randy Johnson in 2002, and Justin Verlander in 2011). The last pitcher to win more games in a season was Denny McLain, with 31 wins in 1968.
In 1991, Welch and New York Times sports columnist George Vecsey co-wrote Five O'Clock Comes Early: A Cy Young Award-Winner Recounts His Greatest Victory, which chronicled Welch's battle with alcoholism that he said started at the age of 16.
"I would get a buzz on and I would stop being afraid of girls. I was shy, but with a couple of beers in me, it was all right."
The book "...marked one of the first times an active professional athlete openly discussed a drinking addiction." An updated version was published after Welch's retirement.