He is best known for his participation in Ultimate Fighting Championship (UFC), and for his professional wrestling career with the World Wrestling Federation and Total Nonstop Action Wrestling. Ken has written two books, Beyond the Lion's Den.

Ken Shamrock competed in the first Ultimate Fighting Championship show in November of 1993 where he progressed to the semi-finals, losing to eventual tournament winner Royce Gracie. Afterwards, Shamrock vowed to avenge his loss to Gracie. However, it would be nearly a year and a half later before he would get his chance. At UFC 5 in May of 1995, Shamrock and Gracie squared off in the UFC's first ever "Superfight." Unfortunately, the match did not live up to expectations. At UFC 5 the UFC had instituted time limits and Shamrock and Gracie fought for the entire allotted time of 30 minutes along with 5 minutes of overtime before the match was declared a draw. Shamrock would go on to fight in further UFC Superfights against Dan Severn, Oleg Taktarov and Kimo Leopoldo. As of 2006, Ken is still making occasional appearances in the UFC.

Shamrock also founded the Lion's Den, a group dedicated to the training of mixed martial arts fighters. Many Lion's Den fighters such as Pete Williams, Jerry Bolander, and Tra Telligman have achieved a modicum of success in mixed martial arts.

Shamrock's Lion's Den has engaged in a feud with former UFC Light Heavyweight champion, Tito Ortiz. Ortiz defeated Lion's Den fighters Jerry Bolander and Guy Mezger, eventually facing Shamrock in one of the highest selling mixed martial arts Pay Per View events of all time in the United States. At UFC 40, Ortiz defeated Shamrock when Shamrock's corner threw in the towel.

April 9, 2005, proved to be a turning point in Shamrock's career and future in mixed martial arts. In the main event, the first ever UFC appearance on basic cable TV, on the popular reality series The Ultimate Fighter finale, Shamrock faced rising star Rich Franklin in a light heavyweight bout. Although Shamrock had more experience and even managed to get Franklin in a painful submission he was defeated by a TKO in the first round when slipping while executing a kick which Franklin took advantage of, thus instantly establishing Franklin as a legitimate threat to other UFC competitors.

On October 24, 2005 Shamrock lost to fellow mixed martial arts legend Kazushi "The Gracie Hunter" Sakuraba in PRIDE: Fully Loaded, by TKO. The decision to stop the fight was considered controversial by some parties, including Shamrock himself, since he was able to bounce up off the canvas right after being knocked down. However, Ken had turned his back to Sakuraba and because he wasn't intelligently defending himself, the referee stopped the fight. His own brother Frank Shamrock commented on the stoppage in an interview with the site Sherdog.com: "…if you're sleeping with your head through the second rope, you're in a bad way. He got clocked. He went down. According to the rules he was no longer defending himself and that's the end of the fight. Was it early? Sure it was early. But was he getting his ass kicked? Yeah. It just depends on how you want to look at it. I tried to look at it abstractly and I saw a guy sleeping with his head through the second rope. That's not good, because if I'm there, please somebody stop this."[3]

On November 19, 2005 at UFC 56, Dana White, the UFC president, announced that Shamrock would be one of the coaches (along with Tito Ortiz) for the upcoming third season of The Ultimate Fighter. At UFC 61 Shamrock and Ortiz will have a rematch of their UFC 40 fight.