Appearing on the first season of Survivor, titled Survivor: Borneo, he was the twelfth of sixteen contestants to be eliminated and was dubbed "The Alphabet Killer" for his famous "Alphabet Voting" strategy, in which he cast his votes based on the order of contestants' names in the alphabet. He later claimed that he chose the system as a way to vote off members of the opposing Pagong tribe, (whose names were all earlier alphabetically than his own tribe) while appearing to do so in a non-deliberate manner.
He has said that he wants to be a writer; after appearing on Survivor he has made acting appearances on The Guiding Light and has appeared as a medical correspondent on various news programs. He was on the news staff of the television station WFOR-TV, a CBS affiliate in Miami, Florida. He was also a CBS Healthwatch Network Medical Correspondent from 2004-2008. He had a weekly 1-minute segment on CBS radio until 2008. In 2000 Dr. Kenniff was a syndicated radio columnist for Winstar Radio (his "Survivor Minute" is heard on 200 stations worldwide), and he contributed a weekly health column to the Miami Herald for several years.
Kenniff established a health news website in 2008, healthapalooza.com. Kenniff began the Dr. Sean Show in 2009, a weekly Saturday medical call-in show on WFTL 850 AM Radio in South Florida.
In May 2010, Kenniff's novel, Etre the Cow, was published, and has received some positive reviews.