Cyclin/cyclin-dependent kinase (CDK) complexes are critical regulators of cellular proliferation. A complex network of regulatory mechanisms has evolved to control their activity, including activating and inactivating phosphorylation of the catalytic CDK subunit and inhibition through specific regulatory proteins. Primate herpesviruses, including the oncogenic Kaposi sarcoma herpesvirus, encode cyclin D homologues. Viral cyclins have diverged from their cellular progenitor in that they elicit holoenzyme activity independent of activating phosphorylation by the CDK-activating kinase and resistant to inhibition by CDK inhibitors. Using sequence comparison and site-directed mutagenesis, we performed molecular analysis of the cellular cyclin D and the Kaposi sarcoma herpesvirus-cyclin to delineate the molecular mechanisms behind their different behavior. This provides evidence that a surface recognized for its involvement in the docking of CIP/KIP inhibitors is required and sufficient to modulate cyclin-CDK response to a range of regulatory cues, including INK4 sensitivity and CDK-activating kinase dependence. Importantly, amino acids in this region are critically linked to substrate selection, suggesting that a mutational drift in this surface simultaneously affects function and regulation. Together our work provides novel insight into the molecular mechanisms governing cyclin-CDK function and regulation and defines the biological forces that may have driven evolution of viral cyclins.