Like his father, former PBA player and Barangay Ginebra Kings slotman Romy "Mama" Mamaril, he is known for his physical and rugged defense.

[3][4] With his play in the FABA gaining attention, he decided to return to the Philippines and pursue a basketball career there.

[8] During the 2004 PBA draft, Mamaril was traded to the Shell Turbo Chargers for Eddie Laure and the seventh overall pick.

[9] During the 2004–05 season, he was among 20 Fil-foreign players suspended for not submitting all the required documents proving their Philippine citizenship.

[12] With Shell taking a leave of absence from the league, Mamaril was traded to the Coca-Cola Tigers for Gec Chia and Edwin Bacani in 2005.

[16] In 2006, Mamaril became part of one of the most controversial trades in PBA history which also involved Rafi Reavis, Rudy Hatfield, Aris Dimaunahan, and Ervin Sotto, as sister teams Coca-Cola and the Barangay Ginebra Kings were not allowed to make trades with one another.

[25] In a loss to Welcoat during the 2007–08 Philippine Cup, Mamaril led the team in scoring with 17 points.

[30] After playing three games at the start of the 2011–12 Philippine Cup, Mamaril had to step away from the team for a while as he had to tend to his wife, who had just suffered a seizure.

[31] In Game 3 of the 2012–13 Philippine Cup quarterfinals against the Rain or Shine Elasto Painters, Mamaril stepped up with 20 points, 10 rebounds, and a career-high six blocks, but their season ended early.

[36] That conference, he helped GlobalPort make its first semifinals appearance after he sent their do-or-die match against Ginebra into overtime, in which his team was able to win.

[41] He then made a bigger impact during the Commissioner's Cup, stepping up when Christian Standhardinger went down with a swollen knee.

[44] With June Mar Fajardo out due to a fractured right tibia and Yancy de Ocampo retiring, the Beermen brought him back for his 18th PBA season.

[54][42][55] Billy is the son of former PBA player Romy "Mama" Mamaril, who played for five teams from 1980 to 1991.

[55] He studied nursing during his last seasons with San Miguel, and even did first aid Fajardo when he injured his leg.