Early career Starting his career in 2015, Day earned an impressive record of 6–1, before he was invited on
Dana White's Contender Series. Day's lone loss in the road prior to DWTNCS came in March 2017 when he suffered defeat to
Bellator MMA and
LFA veteran Nohelin Hernandez. Day's first shot under the ZUFFA lights came against Jaime Alvarez on
Dana White's Contender Series 6 in August 2017. Day came out early with very solid leg kicks but was unable to get the judges’ decision at the end of the bout. After the defeat, Day continued to win on the Hawaiian and California regional scenes. Day won all three of his following bouts, two of them by finishes. The final of the three was against the undefeated Brady Huang. Huang was 8-0 going into the matchup with all eight bouts ending by finish and seven of those eight ending in the first round. The two battled URCC 34: Destiny in August 2018 as the co-main event to former UFC veteran
Tyson Griffin. Day made quick work of the undefeated star, knocking out Huang in the second round. He lost the fight via split decision. Martin Day faced
Davey Grant on July 11, 2020, at
UFC 251. He lost the fight via knockout in the third round. Day fought
Anderson dos Santos on November 28, 2020, at
UFC on ESPN: Smith vs. Clark. He lost the fight via first round guillotine choke. Day faced
Timur Valiev on February 6, 2021, at
UFC Fight Night: Overeem vs. Volkov. He lost the fight via unanimous decision. On February 18, 2021, it was announced that Day had been released from the UFC.
Post UFC In his first appearance after leaving the UFC, Day faced Ramon Taveras on April 30, 2022 at Combat Night Pro:Jacksonville. He lost the bout via guillotine choke in the first round. ==Championships and accomplishments==