This article examines the interaction of different modes and levels of legitimacy within network governance institutions over time. Drawing on new theoretical directions in European governance studies and empirical findings from Canada, we contend that whereas input legitimacy can be exchanged, or traded-off, with output legitimacy to reinforce the overall legitimacy of a network governance institution, “throughput legitimacy” functions as a necessary condition that sustains legitimacy over time. Through a comparison of homelessness governance networks in Toronto and Calgary, we find that throughput legitimacy carries an amplification effect that results in either virtuous or vicious cycles. That is, we argue and demonstrate that low throughput legitimacy in network governance institutions can effectively bring down the whole house of cards.