From 2009, Kasun served as a secular priest in the Archdiocese of Edmonton, ministering at two inner-city parishes in Edmonton.[5] As pastor in Edmonton, Kasun sought to increase fundraising for the local homeless population and advocated for the Sudanese diaspora to maintain celebration of Mass in Dinka and with their cultural elements so long as they followed the General Instruction of the Roman Missal.[6][7] Kasun also work on an abuse prevention program in Edmonton, facing a backlash from parishioners for implementing "strict" rules that required windows in every room and multiple adults in Sunday school classes. Efforts by Kasun and others led to the Archdiocese of Toronto receiving Praesidium Accreditation certifying a high standard of abuse prevention.[8]
Kasun was officially welcomed to the archdiocese through a Mass at St. Michael's Cathedral, Toronto, on 6 October 2016.[12] As an auxiliary bishop, Kasun has presided over liturgies on behalf of the Chinese Canadians and Goan Catholics in the archdiocese.[13][14] During the COVID-19 pandemic, Kasun and fellow auxiliary bishop Vincent Nguyen filled the responsibility of ministering to the northern portion of the archdiocese in the absence of an additional auxiliary bishop.[15]