McWilliams was born along his twin brother, McJoe, in the municipality of Ceiba in the northeast coastal region of Puerto Rico's main island.

At the age of 12, they entered Gimnasio Fito Ramos, a gymnasium located in the municipality of Fajardo, Puerto Rico.

[3] They initially took up the sport as a hobby, but after six fights they began establishing a pattern of only spending a limited amount of time inactive during a month, visiting the gymnasium at least every two weeks.

[7] After advancing to the finals, he earned the tournament's gold medal, winning his contest against Odilion Zaleta of Mexico by points (14:8).

[8] Prior to the 2007 Pan American Games, both of the brothers moved up one division in order to avoid being matched against each other in local tournaments.

Arroyo, now fighting at the flyweight division, debuted in the tournament's first date, defeating Lucas Navarro of Argentina by AB in the second round.

The Arroyo twins, McWilliams and McJoe, qualified to the 2008 Summer Olympics due to their performance at the AIBA World Championships.

[12] In respect, Arroyo expressed that he would make his decision after comparing the offers provided by the government and professional promoters.

[13] The twins, José Pedraza and Jonathan González began their preparations in a massive sports complex located in Salinas, Puerto Rico, named Albergue Olímpico.

[14] His next competition was the Torneo Nacional de Boxeo Aficionado Isaac Barrientos, Puerto Rico's national championship tournament.

Arroyo debuted in the tournament's third date, defeating Yoon Kyoung Min of Korea by RSCH in the second round.

Arroyo won the tournament's flyweight gold medal, defeating Tugstsogt Nyambayar of Mongolia with a score of 18:2, the widest margin in all of the finals.

[26] On December 8, 2009, the Arroyo brothers issued a press release through their mother, Milagros Acevedo, informing that they were no longer interested in continuing their amateur careers.

[27] In the letter, they report differences with the criteria used to provide economical support to high-performance athletes, as well as schedule conflicts with their original plans to pursue a professional career after the 2010 Central American and Caribbean Games.

[27] These concerns were restated in a subsequent press conference, where they noted that supporting their respective families was the main reason to pursue a professional career.

[28] Arroyo's debut was scheduled for February 27, 2010, taking part of a card titled "Haciendo Historia", where Wilfredo Vázquez, Jr. fought Marvin Sonsona for the super bantamweight world championship.

[29] The fight concluded after 1:46 of the first round had passed, when Arroyo connected a single punch that left Sánchez unconscious for several minutes, requiring paramedical help and an oxygen mask, this result earned him a victory by knockout.

[31] His next opponent was Samuel Gutiérrez, who was forced on the defensive from the opening round, receiving several combinations and suffering a broken nose.

[32] In his next fight, he suffered an upset loss to Takashi Okada (1-0-1) who knocked Arroyo down in the second round en route to a unanimous decision victory.

After repeating his previous performance in a rematch with Casillas, Arroyo defended the championship by knocking Ronald Ramos out in five rounds.

[33] His return took place in an International Boxing Federation title eliminator on June 19, 2014, where he defeated Froilan Saludar by knockout in two rounds.

Initially Arroyo was supposed to face Julio Cesar Martinez, who pulled out of the fight just days before because of hand injuries.