Originally from neighboring Betis, Guagua, Castro was a part of the high school basketball team of Don Honorio Ventura State University in Bacolor.

Things changed on the 2004 season in which PCU was able to make a turnaround with Castro, Gabby Espinas and Robert Sanz at the helm, denying Letran a back-to-back championship run.

In 2006 season, Castro gave out an all-NCAA team performance as he was named as one of the Mythical 5 members; he and Espinas led PCU to a third consecutive Finals appearance this time against San Beda College.

The Dolphins would be suspended the following year after irregularities on their high school team, thus ending Castro's collegiate career.

The Fil-Am Jayson Castro William was headed to the AMP Mobile Phone team, which he was a backup to LA Tenorio.

His PCU teammates Gabby Espinas and Rob Sanz, gave their worth talents to remain in the Philippine Basketball League.

They were joined by Ateneo Blue Eagles team, composed of Paolo Bugia, LA Tenorio, Magnum Membrere, and company.

He was with Mac Cardona and his former PCU teammates Gabby Espinas, Rob Sanz, Beau Belga, and Joel Solis.

After PCU's championship, Lamoiyan Corporation sponsored the core of that team to participate in the PBL as the Hapee-PCU Teeth Sparklers.

Hapee made several more playoff runs but were beaten either by Harbour Centre or by Toyota-Otis (ironically mostly composed of Letran players).

In the midst of Jayson Castro's third season at Hapee, he was acquired by Junel Baculi, his coach in PCU, to join the RP team with Marvin Cruz, JC Intal, Ryan Araña, Chad Alonzo, and Beau Belga, with 2 imports.

He was with Gabe Norwood, Beau Belga, Chad Alonzo, Eugene Tan, Patrick Cabahug, and Jeff Chan.

[6][7] However, about a month before the start of the season, the Slingers withdrew permanently from the NBL citing excessive transport costs to Australia.

He has been the Gilas Pilipinas main gunner in the tournament, with his partnership with naturalized player Andray Blatche proving to be a formidable duo on the offensive end.

Castro was among the players who made it into the Final 12 roster of Gilas Pilipinas for the Men's Olympic Basketball Qualifying Tournament held in Manila.