Wagner's law is widely held as the first model of public expenditure in the history of public finance. It is an antithesis of the Keynesian postulation that Government expenditure causes economic growth hence taking the former as a policy instrument. This study contributes to the existing literature by assessing the empirical evidence of Wagner’s law in Kenya for the period of 1960-2009. It applies cointegration analysis in the investigation of long-run relationship between public expenditure and GDP. The existing literature reveals that Cointegration is a necessary condition to establishing a long-run relationship between public expenditure growth and income. However, in support of the Wagner's law, the required sufficient condition is the existence of a unidirectional causality from GDP to public expenditure. The study employed the Engle and Granger twosteps cointegration test, Granger causality test and time series aggregated data to carry out the test. The findings reveal that two versions of the law meet the necessary and sufficient condition hence, the Wagner’s law holds in Kenya for the entire period under study.