Abstract: In what respects does military service matter in the legislative behavior observed in the U.S. Senate? While scholars long posit that the personal backgrounds of political elites shape the legislative behavior of U.S. Senators, relatively little is known to what extent military service influences legislative collaboration and effectiveness. We argue that pre-legislative military service, an intensive personal and professional experience, socializes potential legislators to be effective lawmakers by providing them with critical leadership skills, and a sense of collaboration, needed to work constructively towards legislative ends. Drawing on original data capturing the military background of U.S. Senators from the 93rd to 115th Congresses (1973-2018), we find strong evidence that veteran Senators collaborate more on proposed legislation than non-veterans, particularly in bipartisan pairs across differing policy issues. Secondly, we find legislation with more veteran sponsors make it further in the legislative process from being reported out of committee to ultimately becoming law. Considering these findings, we test our theoretical mechanism and find evidence that military service is correlated with more effective and legislatively productive Senators. Taken together, we show that pre-senatorial careers in public service shape collaboration and effectiveness in the contemporary U.S. Senate.