He had many run-ins with law enforcement until eventually he went to live with Bob Shamrock, who with his wife Dede, had taken in thousands of troubled boys (including Frank's older adoptive brother Ken).

He became a member of Ken's training school, the Lion's Den, and made his mixed martial arts debut in the Pancrase organization in Japan.

Shamrock faced off against expert grappler and eventual tournament finalist Manabu Yamada later that night, but was defeated via submission at 8:38 of round 1.

In 1995, Shamrock would get a win over Katsuomi Inagaki, but he faced next his own trainer, MMA legend and Pancrase co-founder Masakatsu Funaki, who defeated him.

Shamrock then devised a new strategy and started mocking and taunting Rutten during a leglock exchange, in order to get the Dutch fighter angry enough to commit an infraction.

The plan was successful, and Frank won a point when Rutten gained a yellow card for hitting him with a closed fist, but Shamrock still lost the match by decision.

Shamrock faced Olympic alternate wrestler and master submission grappler Minoru Suzuki on January 28, 1996, for the vacant belt in a match that drew widespread anticipation.

In an epic bout, Shamrock submitted Suzuki with a kneebar at the 22:53 mark of the fight to win the King of Pancrase interim title in front of a sellout crowd in Yokohama.

Shamrock scored decision wins over Ryushu Yanagisawa and Osami Shibuya before facing off against Bas Rutten for the third time for the undisputed King of Pancrase title.

Rutten won the bout via TKO due to a cut stoppage when a tired Shamrock received a knee to the forehead in a takedown attempt.

[8] Shamrock then avenged his loss to Manabu Yamada in his next bout, scoring an impressive submission win over the talented grappler.

Within the Lion's Den, Shamrock trained up-and-coming stars such as Jerry Bohlander, Pete Williams, and Guy Mezger.

Cornered by The Alliance, Shamrock fought Enson Inoue in a bout in Vale Tudo Japan '97 that would determine who would fight Kevin Jackson for the newly created UFC Middleweight Championship.

Shamrock then made his first title defense against then undefeated Extreme Fighting champion Russian Igor Zinoviev at UFC 16.

Zinoviev was a feared Russian kickboxer and sambo specialist who held wins over Mario Sperry and Enson Inoue.

[10] Zinoviev's fight with Shamrock forced him to retire permanently from mixed martial arts ending the Russian's career.

In October 1998, Shamrock avenged his earlier loss to John Lober by beating him decisively in 7 minutes at UFC Brazil.

[11] However, in what is widely considered to be one of the greatest fights in UFC history, Shamrock won after brutal elbows, punches, and hammer blows forced Ortiz to tap out at the end of round 4.

He signed a deal to fight Brazilian Jiu-Jitsu standout Elvis Sinosic at K-1, the premiere kickboxing event in the world.

However, Gracie is an elite Brazilian Jiu-Jitsu instructor with years of training under his belt as well being the mentor of students such as the Diaz brothers, Nate and Nick.

On September 14, 2006, it was announced that Shamrock had signed a multimillion-dollar contract with startup MMA organization "The World Fighter" and was scheduled to fight in January 2007.

During the fight, Gracie repeatedly took Ken to the ground, but Shamrock kept active by stopping his positional advance and hitting knee strikes from the bottom.

Dean had already warned Shamrock once earlier in the fight about striking to the back of the head—an illegal move under the Unified Rules of Mixed Martial Arts.

On January 11, 2008, it was announced that Shamrock would face Cung Le in a match on March 29, 2008, for the Strikeforce Middleweight Championship during the joint Strike force-Elite XC event at the HP Pavilion in San Jose, California.

At one point in the later stages of round 3 Shamrock appeared to have Le hurt but was unable to finish him with a barrage of punches against the cage.

Shamrock was unable to answer the bell to begin round 4 due to a broken arm caused by one of Le's kicks and the fight was ruled a TKO as a result of corner stoppage.

[18] On October 15, 2017, Shamrock fought Pride veteran Kazushi Sakuraba in a grappling exhibition match at the 2017 Rizin World Grand Prix Opening Round in Fukuoka, Japan.

[8] Shamrock's improved cardio, along with an excellent defensive guard work learned from Tsuyoshi Kohsaka, enabled him to survive against superior opponents in order to seek for openings, which he capitalized on with aggressive strikes and submissions.

[28] Shamrock is also featured in the award-winning documentary on the sport of mixed martial arts titled Fight Life, released in 2013.

[32] In January 2017 Shamrock launched The Bakeout,[33] an online talk show seeking to "uplift, unite, educate, challenge, and encourage viewers in a manner consistent with the teachings of the healing powers of cannabis".