Keenan Cornelius was born on February 25, 1992, in Hawaii, USA. His initial exposure to martial arts came through his father, Tom Callos, a 6th-degree black belt in Tae Kwon Do. Despite this early influence, Keenan showed minimal interest in martial arts during childhood, discontinuing training shortly after starting. At age 14, however, he rediscovered martial arts, this time through Brazilian Jiu-Jitsu.

Cornelius began his Brazilian Jiu-Jitsu journey at the BJ Penn Academy in Hilo, Hawaii. After two years of dedicated training, he earned his purple belt under BJ Penn. Seeking a more structured training environment due to limited gi instruction and the absence of full-time black belt instructors, Keenan consulted with friend Andris Brunovskis and subsequently joined Lloyd Irvin's academy in Maryland. Under Irvin's mentorship, Cornelius rapidly evolved into one of the most dominant competitors at the purple and brown belt levels. He notably achieved the prestigious "Jiu-Jitsu Grand Slam," capturing IBJJF World, Pan, European, and Brazilian National titles.

In early 2013, following controversies surrounding Lloyd Irvin and his academy, Keenan departed from the team. He soon joined Atos Jiu-Jitsu, training under the guidance of André Galvão. Galvão awarded Cornelius his black belt on September 14, 2013. Representing Atos, Keenan continued to excel internationally, securing multiple IBJJF titles and podium finishes at elite competitions, including the IBJJF World Championship and the ADCC World Championship.

In April 2019, Keenan parted ways with Atos due to differences in athlete-coach dynamics, particularly regarding training structure and discipline. Shortly thereafter, he founded Legion American Jiu-Jitsu (AJJ) in San Diego, California. Legion AJJ quickly gained recognition for its innovative and open-minded training philosophy, emphasizing experimentation, creativity, and humor. Keenan positioned Legion as a welcoming hub for grapplers seeking alternatives to traditional Brazilian Jiu-Jitsu team structures, famously branding his style as "American Jiu-Jitsu."

Cornelius is widely regarded as one of Brazilian Jiu-Jitsu's most innovative and cerebral athletes. He developed and popularized lapel-based guard systems, notably the Worm Guard and Ringworm Guard, significantly reshaping modern guard play. His advocacy for rule changes within IBJJF competitions, particularly the legalization of heel hooks and reaping in no-gi divisions, contributed directly to the organization's eventual rule updates in 2021. Keenan emphasizes strategic gameplanning and detailed tape study, regularly sharing insights through his podcast, "Dojo Storm."

Keenan Cornelius has become a prominent advocate for the term "American Jiu-Jitsu," highlighting the historical roots of jiu-jitsu in the United States dating back to Mitsuyo Maeda's teachings in the early 1900s. He humorously yet earnestly incorporates American symbolism, such as the eagle and the American flag, into his branding. His stance has sparked public discussions about the evolution and identity of modern jiu-jitsu, resonating with many non-Brazilian practitioners worldwide.

Although primarily focused on grappling, Keenan has explored cross-training in mixed martial arts. In 2020, he collaborated with UFC fighter Dominick Cruz, exchanging grappling instruction for striking lessons, humorously documenting their sessions on social media. Cornelius has expressed openness to potentially competing in MMA, provided it aligns with his responsibilities at Legion AJJ. Reflecting his vision of an "American" grappling style, he integrates wrestling and judo fundamentals into Legion's curriculum.

Cornelius is renowned for his cerebral and innovative approach to competition. His matches frequently feature lapel-based guards and unconventional techniques designed to disrupt traditional passing strategies. Keenan prioritizes meticulous gameplanning and strategic preparation, often extensively studying opponents to exploit specific weaknesses and maximize competitive advantages.

Keenan Cornelius has significantly reshaped modern guard play through his technical innovations and strategic insights. His contributions have directly influenced IBJJF rule changes and broader acceptance of previously restricted techniques. By establishing Legion AJJ, he created a prominent and influential academy that continues to impact the global Brazilian Jiu-Jitsu community, solidifying his legacy as one of the sport's most innovative and influential figures.