This paper examines the effect of corporate governance mechanisms on companies’ decision to assure their sustainability reports and their choice of assurance provider in countries with a greater stakeholder orientation. The corporate governance factors analyzed are related to the strength of the board’s monitoring function, which is determined by the size, independence and activity of the sustainability committee. The international sample consists of 610 companies operating in countries that are more stakeholder oriented from the period 2007–2014. We propose alternative logit models of analysis using the random effects estimator. Consistently with our propositions, our results provide evidence that a firm’s sustainability assurance and its choice of the accounting profession are positively associated with board independence and the activity of the sustainability committee. In addition, the empirical evidence obtained shows a U-shaped relationship between the board size and assurance issues.