[2] Nagashima joined the team of Nippon Kempo and started training seriously when he was matriculated in University of Marketing and Distribution Sciences.

He won ten consecutive victories to start his career, fighting for Deep, Mars and New Japan Kickboxing Federation.

He participated K-1 World MAX 2009 Japan Tournament on February 23, 2009, and he knocked out Hayato at 2R at the quarterfinal, but he was beaten by Yuya Yamamoto and lost by TKO because of being cut at the semifinal.

[5] On December 31, 2010, Nagashima took part in a "mixed rules" bout against Judo and Brazilian Jiu-Jitsu expert Shinya Aoki at Dynamite!!

Aoki took advantage of the fact that he only had to survive through a single three-minute round of Kickboxing and thus threw pointless attacks like flying dropkicks and rolling heel kicks while holding onto the ring ropes in order to run the clock down.

[6] He faced Henri van Opstal at REBELS 13 in Tokyo on October 28, 2012 with a place at the Shoot Boxing World Tournament 2012 on the line.

[7][8][9] He lost a one-sided unanimous decision to Robin van Roosmalen at Glory 4: Tokyo - 2012 Heavyweight Grand Slam in Saitama, Japan on December 31, 2012 after being dropped in round one.

Thereafter, Yuichiro began competing actively for IGF, achieving significant victories over Munenori Sawa and Bob Sapp.

He entered the match wearing the first anime cosplay he wore in his career, that of Haruhi Suzumiya from the eponymous franchise, and suggested this was his last fight.

[19][20] On September 23, 2021, Nahashima was scheduled to face Shohei Asahara, Yasuomi Soda and Yusaku Nakamura in his retirement exhibition match at DEEP☆KICK 55.

[21] As in the previous occasion, he came to the match wearing a significant cosplay, the same Miku Hatsune costume he wore in his K-1 championship in November 2008, and thanked the fans for their support in his 14 years career.