On August 23, 2008, just a year after turning pro, Casimero defeated Rodel Quilaton for the then vacant Philippines Boxing Federeration (PBF) flyweight title, winning a unanimous decision (UD) through ten rounds.
[6] This win can give Casimero a title shot against Iván Calderón of Puerto Rico, who previously defeated Filipino boxer Rodel Mayol.
[7] Casimero fought Ramón García Hirales on July 3, 2010, for his first defense of the interim WBO light flyweight title but lost by split decision.
[9] On February 11, 2012, Casimero, and members of his team were kicked and assaulted when fans in Mar del Plata hurled chairs and stormed the ring following his 10th-round knockout of local Argentine fighter Luis Alberto Lazarte for the IBF interim junior flyweight championship.
[11] On July 19, 2012, Casimero was upgraded to the status of full IBF junior flyweight champion because of the inability of Ulises Solis to defend his title on or before October 30.
On June 27, 2015, he lost his return to flyweight bout via unanimous decision against IBF champion Amnat Ruenroeng in a controversial match.
[16] A rematch on May 25, 2016, Casimero challenged Ruenroeng again for the IBF flyweight title and this time he knocked Ruenreong out with a vicious left hook to the body in the fourth round to become a two-division world champion.
[17] In his second title defence, Casimero faced undefeated Charlie Edwards, on September 10, 2016, on the undercard of Gennady Golovkin vs. Kell Brook at The O2 Arena in London, England.
Edwards beat the count, but as soon as he got up, Casimero unleashed a flurry of punches towards the challenger, which prompted the referee to end the fight immediately.
Despite Tete being the favorite for the fight, Casimero scored a third-round knockout over the long-reigning champion to capture his WBO bantamweight title.
[23] Casimero have scheduled to face undefeated unified bantamweight champion Naoya Inoue in a unification clash on April 25, 2020, but the COVID-19 pandemic caused the fight to be cancelled.
[31] Casimero had been scheduled to fight his mandatory challenger and former IBF bantamweight champion Paul Butler on December 11, 2021, at Coca-Cola Arena in Dubai, United Arab Emirates.
Later on, Casimero officially signed a contract under Treasure Boxing Promotion with the Japanese startup led by former world champion and CEO Masayuki Ito in February 2023.
[37] Casimero made a successful homecoming, dethroning Namibia's Fillipus Nghitumbwa via unanimous decision win to claim the WBO Global super bantamweight title at the Okada Manila in Paranaque City on May 13, 2023.
[46][47] Casimero owns a dog named Dan Tour Vlogs who constantly defends his master on social media against haters and real boxing fans.