The oft-quoted phrase is originally attributed to
baseball manager Leo Durocher.
In 1946, the New York Giants, managed by Mel Ott were in last place. Durocher was managing the crosstown Brooklyn Dodgers at the time and was asked about the Giants. He commented that Ott and his team were "the nicest guys in the world. And where are they? In last place!" (another source quotes it as "The nice guys are all over there. In seventh place!"; Yet ANOTHER source says Durocher said "all nice guys. They'll finish last. Nice guys. Finish last").
Regardless of the actual words, the next day sportswriters misquoted him, or altered his quote, changing it to "nice guys finish last". The quote is often used in sports circles to encourage toughness, and also in dating circles (as in the other writeups in this node).
An interesting offshoot of this comes in the titles of autobiographies by both Durocher and Hall of Famer Monte Irvin (who played under Durocher for several years). Durocher named his book (1975) "Nice Guys Finish Last". Irvin titled his book (1996) "Nice Guys Finish First".