Policies are management instruments that organizations employ to ensure their stakeholders and others understand their guiding principles. This paper examined the importance and impacts of policy coherence, mandate overlaps and policy management towards achieving policy effectiveness. The authors examined some extant development policies in Nigeria for policy coherence and assessed some agencies that implement these policies for mandate overlaps and mandate clarity/exclusiveness. To reduce policy incoherence and mandate overlaps, the paper recommended that policy makers should consult widely to understand all the issues involved and use multi-sector teams to ensure the development of effective policies that are robust in nature. Finally, it canvassed that policies be reviewed or evaluated from time to time to strengthen identified areas of weaknesses and enhance their effectiveness.