Jansen returned to the Pacific Coast League as a player-coach with Seattle (1955 and 1957) and Portland (1958–60). After a call from former teammate Alvin Dark, Jansen returned to the Major Leagues as pitching coach for the San Francisco Giants in , with Dark as manager. Jansen remained as pitching coach for eleven seasons, and helped to develop future Hall-of-Famers Juan Marichal and Gaylord Perry. During his tenure, the Giants made appearances in the 1962 World Series and 1971 National League Championship Series. He then moved on to his final MLB coaching job, handling pitchers for the Chicago Cubs in –73, working for his old Giants manager, Leo Durocher, and then former teammate Whitey Lockman.