While representing Webster Groves High School in a St. Louis-area all-star game, he attracted the attention of Doc Hayes, the men's basketball coach at Southern Methodist University in Texas.

He scored 1,753 points in his three varsity seasons (1954–1957), led the Mustangs to three consecutive Southwest Conference championships, and was named to three All-SWC teams.

"[1] In 1956, Krebs' team reached the NCAA National Semifinals, where they faced Bill Russell and the University of San Francisco.

The next year, Krebs earned consensus All-American honors and appeared on the cover of Sports Illustrated with the headline: "Big Jim and the Texas Boom".

His team reached the NCAA Tournament once again that spring, but they were defeated 73–65 by Wilt Chamberlain and the University of Kansas in the Midwestern semifinals.

Other schools in the Southwest Conference, who had mainly emphasized college football, took notice of Krebs' success and increased funding for their own basketball programs.

He played with the Lakers (who moved to Los Angeles in 1960) for seven seasons, mostly in a reserve role behind Vern Mikkelsen, Larry Foust or Rudy LaRusso.

[6] Krebs also engaged in several fights with opposing players, including Boston's Bill Russell[7] and St. Louis' Bob Pettit.