Walker started and filled various relief roles coming out from the bullpen as a closer, middle reliever, and set-up man as well. His most productive season statistically came in 1970 with Pittsburgh, when he finished 15–6 (3–1, three saves in relief), while his 3.04 ERA, .714 winning percentage, and 7.1 hits per nine innings all ranked him third among National League pitchers. He also fired a pair of two-hit shutouts. That year, the Pirates won the National League East title for their first post-season berth since winning the 1960 World Series. However, they were swept by the Cincinnati Reds in the NLCS. Walker was the losing pitcher in Game Two, giving up two runs (one unearned) in seven innings in a 3–1 Reds victory. Bobby Tolan scored all three Reds runs, including a home run off Walker in the fifth inning.