The activities of the construction industry are vital to the achievement of national socio-economic development goals of providing shelter, infrastructure, and employment. The sector accounts for five percent of the gross domestic product (GDP) in Kenya and employs about 13 percent of the country’s total workforce. The devolved units (counties and constituencies) have invested the substantial amount of public funds in infrastructure development and yet social audit and media reports have raised the concern over the poor quality of the projects undertaken by these devolved units. This study, therefore, sought to investigate the contractors’ operations in the quality assurance process and how they affect the quality of the public building projects. A case involving sixty-four projects sampled from Busia County and its seven constituencies was carried out. The study was a sample survey whose data was collected using questionnaires, interviews and physical observation exercise. The data were analyzed using descriptive statistics and Chi-square good-for-fit tests. The findings indicated the insufficient participation of the contractors as quality assurance providers in the construction process of the public building projects undertaken by devolved units. The participation rate was at 43% which implied that the projects were constructed with inadequate checks on quality through the assurance process. The physical observation assessment revealed the quality of the public building projects stood at 34% compliance to building specifications and standards which is inadequate to meet the required standards. It is concluded that the involvement of contractors in the quality assurance process in the construction management process of public building projects undertaken by the devolved units was inadequate and impacts negatively on the quality of the projects. In order to achieve the sustainable development as far as the infrastructural development is concerned, the involvement of contractors as quality assurance providers in the construction process was critical. This therefore called for the development of the legal and quality frameworks to guide the construction sector at the devolved units.