As the director of military security, Mậu was a participant in the 1963 South Vietnamese coup that deposed and executed President Diệm and his brother and chief adviser, Ngô Đình Nhu. The ruling Ngô family came under pressure in the Buddhist crisis of 1963, when discontent among the country's Buddhist majority towards the pro-Catholic regime erupted into civil unrest. There were many conspiracies against Diệm in 1963, many of them by different cliques of military officers independent from one another. According to Ellen Hammer, there were "perhaps as many as six and possibly more" different plots, and these spanned the gamut of society to include civilian politicians, union leaders, and university students.