Frenn took up track and field in 1957 at
North Hollywood High School, initially competing as a runner rather than a thrower. He got interested in hammer throwing in 1959 after seeing Olympic champion and world record holder
Hal Connolly throw in a local meet. He improved rapidly, placing sixth at the 1962
NCAA championships with ; in that meet, he represented
Abilene Christian University, a school he only briefly attended. Nevertheless, he failed to gain selection either to the
Pan American Games or the dual meet against the Soviet Union in Moscow. although with the emergence of
Ed Burke as a third top thrower alongside Hall and Connolly, making the Olympic team would in any case have required Frenn to improve his personal best considerably. and then his leg just before the 1966
national indoor championships, where he would have competed in the 35-lb
weight throw. Nevertheless, Frenn took it up that year after a friend mailed him a free weight; in his first competition with the outdoor weight he threw 45 ft in (13.98 m), which would have broken
Bob Backus's world best of from 1957, but his implement was found to have been slightly too light. Frenn also improved in the hammer, throwing in
Sacramento on June 10 for a new personal best; at the national championships he threw and placed second as the winner, Burke, set a new
American record of . In 1968 Frenn placed second at the national championships with a throw of , In November, after the Olympics, Frenn improved his personal best to in
Long Beach. In 1969 Frenn threw the 35-pound indoor weight 73 ft in (22.33 m) to beat Connolly's world best from 1960; he also broke his own world best in the 56-pound weight throw that year, improving to . In the hammer, Frenn placed second in the AAU meet for the third consecutive year; despite throwing , he still lost to Gage by a foot and three inches. That summer he also won his first national title in the hammer throw, throwing and beating Gage by a foot and seven inches; in practice before the championships he reportedly threw , which would have been a world record if duplicated in competition. His best in official meets was , which he reached in
Berkeley on May 30; that mark was then the third-best by an American, and would remain his career best. He appeared on the cover of the
Sports Illustrated's July 6, 1970, issue. In 1971 Frenn repeated as national champion in both the hammer and the indoor weight and was ranked the #1 hammer thrower in the United States for the third consecutive year; At the
1971 Pan American Games in
Cali he won silver in the hammer with a throw of , losing to Hall by only six and a half inches. In 1972 he won his third consecutive title in the indoor weight, breaking his championship record from the previous year with a throw of . Later that winter he threw 74 ft in (22.62 m) in a dual meet against the Soviet Union to break his own world record. That summer Frenn finally qualified for his first Olympics, albeit narrowly; with his throw of , he placed third at the 1972 Olympic Trials and got the last spot on the team behind Gage and
Al Schoterman. At the
Olympics in
Munich he only reached , placing 27th in the qualification round and failing to qualify for the final. and in 1976, when he was a close second behind
Larry Hart. In 1975, he competed as a hammer thrower in his third
Pan American Games, but only placed seventh with a throw of . ==Other sports==