Early career Brennan made his professional mixed martial arts debut in 1996 and compiled a record of 4–1–1 before being signed by the UFC.
UFC Brennan made his UFC debut against Courtney Turner at the
UFC 16 Lightweight Tournament on March 13, 1998, and won via
armbar submission in the first round. In his next fight later that night, Brennan lost to future five-time champion,
Pat Miletich via submission.
King of the Cage Brennan made his
King of the Cage debut against future
King of the Cage Lightweight Champion Joe Stevenson and won via submission. In his next fight, Brennan fought against future
MFC Lightweight Champion Antonio McKee for the
King of the Cage Middleweight Superfight Championship and won via
knockout with a head kick. In his first title defense, Brennan was defeated by Joe Hurley after he was knocked out from elbows. On August 4, 2001, Brennan defeated Kevin Hogan via armbar submission to become the new
King of the Cage Middleweight Champion and then in his next fight, successfully defended his title against
Steve Berger. After a one-fight stint with the UFC at
UFC 35, a loss to
Gil Castillo, Brennan won two more fights outside of
King of the Cage before returning to defend his title again, this time against
John Alessio. Brennan was defeated via
TKO.
PRIDE Brennan went 0–2 in Shooto, losing a controversial decision to Takanori Gomi before he made his
PRIDE debut on October 5, 2003, at
Bushido 1 against
Eiji Mitsuoka and won via
kimura submission in the first round. In his next appearance for the organization, he faced
Daiju Takase a year later at
Bushido 2 and lost via unanimous decision. His last fight in PRIDE was at
Bushido 12 against
Tatsuya Kawajiri, and he was defeated via TKO only 29 seconds into the first round.
Post-PRIDE Since his last fight in PRIDE, Brennan has gone 2–2, competing for Cage Rage and King of the Cage. ==Championships and accomplishments==