From 1926 to 1938, Horrell was an assistant coach for the UCLA Bruins. He then served as the head coach from 1939 to 1944, compiling a 24–31–6 record. His 1942 UCLA Bruins team lost to Georgia in the 1943 Rose Bowl. He was the first coach to lead a UCLA team to defeat the rival USC Trojans. It was UCLA's first football victory in the UCLA–USC rivalry.