Imanari is a successful submission wrestler, placing as runner-up twice at the All-Japan Combat Wrestling Championship.

[3] After a childhood marked by a spinal condition which required surgery three times, Masakazu had his first contact with combat sports at age 18, training kickboxing and catch wrestling at the Fujiwara Dojo by Satoru Sayama's mediation.

Imanari gained popularity during his mixed martial arts career in Japanese promotion ZST fighting the likes of top Lightweights including Dokonjonosuke Mishima, Jorge Gurgel and Marcus Aurélio despite being a much smaller Featherweight.

He defeated Gurgel, who is a Brazilian jiu-jitsu black belt former UFC fighter via catch wrestling staple submission (heel hook) while being lighter in weight.

This accomplishment made him the only mixed martial artist in the Featherweight division to hold belts in two separate major promotions (DEEP and Cage Rage).

In the semifinal round at Fight For Japan: Genki Desu Ka Omisoka 2011, Imanari lost to Antonio Banuelos via split decision.

He faced unbeaten URCC Flyweight Champion Kevin Belingon at ONE Fighting Championship 3 at the Singapore Indoor Stadium on March 31.

After over a year away from the sport, Imanari returned to face Yuta Nezu at Road to ONE 3: Tokyo Fight Night on September 10, 2020.

Imanari faced Kenta Takizawa in the opening round of the Bantamweight Grand Prix at Rizin 29 on May 30, 2021.

[24] Imanari is primarily a grappler, and is universally known for his skill and preference for leglocks,[25] which gained him the nickname of "Ashikan Judan" ("The Great Master of Leg Submissions").

Though his striking game is considered inferior,[25][26] Imanari often engages in stand-up battles in order to bait the opponent to the ground, usually by dropping down after or while seizing control of his leg[26] or by way of ashi garami.

[27] He stands out for his dexterity in toehold and heel hook variations, and is an avid user of the 50/50 guard in order to transition between submission attempts.

[26] He describes his signature leglock style as not based in sambo as it is popularly believed, but "purely self-taught",[4] and explained his preference for it as "because it hurts the opponent".