In seven seasons at Cerritos, Killingsworth compiled a 157–46 record and led the Falcons to the 1968 California junior college championship.

In March 1971, he was hired as head coach at Idaho State in the Big Sky Conference, succeeding Dan Miller.

The 1976–77 season was his last at ISU and was capped off by a run to the Elite Eight of the NCAA tournament, following a one-point upset of UCLA in the Sweet Sixteen in Provo, Utah, behind the play of seven-foot (2.13 m) center Steve Hayes.

[10][11] After two losing seasons, the first in his career at any level, he left the Cowboys to take the head coaching job at TCU in the Southwest Conference (SWC).

[12][13] Nicknamed "Killer" during his time at Fort Worth, Killingsworth raised the profile of a school that had experienced limited basketball success in recent seasons.