Whakaotihia te rerenga e whai ake nei: I hoki mai a Jansen ki te Pacific Coast League hei kaitakaro-kaiako me Seattle (1955 me 1957) me Portland (1958<unk>60). Whai muri i te piiraa mai i tana hoa o mua a Alvin Dark, ka hoki a Jansen ki nga Riiki Nui hei kaiako tuku mo nga San Francisco Giants i te tau 1997, me te pouri hei kaiwhakahaere. I noho a Jansen hei kaiako tuku mo nga tau tekau ma tahi, a i awhina ia ki te whakawhanake i nga Hall-of-Famers a Juan Marichal me Gaylord Perry. I a ia e mahi ana, i puta nga Giants i te 1962 World Series me
Ko te 1971 National League Championship Series. I neke atu ia ki tana mahi whakangungu MLB whakamutunga, e whakahaere ana i nga peera mo te Chicago Cubs i te 73, e mahi ana mo tana kaiwhakahaere Giants tawhito, a Leo Durocher, ana ko tana hoa o mua a Whitey Lockman.