Tusi se faaauauga mo lenei palakalafa - Na toe foʻi Jansen i le Pacific Coast League o se tagata taalo-faiaoga ma Seattle (1955 ma le 1957) ma Portland (1958<unk>60). Ina ua maeʻa se valaʻau mai le sa avea muamua ma 'au a Alvin Dark, na toe foʻi Jansen i le Major Leagues e avea ma faiaoga faʻatonu mo le San Francisco Giants i le, ma Dark e avea ma pule. Na tumau pea Jansen o se faiaoga mo le sefulu ma le tasi vaitau, ma fesoasoani i le atinaʻeina o le lumanaʻi Hall-of-Famers Juan Marichal ma Gaylord Perry. I le taimi o lona nofoaiga, na faia ai e le Giants ni faaaliga i le 1962 World Series ma le 1971 National League Championship Series
. Na ia siitia atu i lana galuega faiaoga MLB mulimuli, taulimaina pitchers mo le Chicago Cubs i <unk>73, galulue mo lona tuai Giants pule, Leo Durocher, ma lea sa avea muamua au Whitey Lockman.