Preston Leslie was admitted to the bar and began his career as a county attorney. He transitioned from farming to a political career, serving in various state legislative roles. Leslie accumulated experience in both the state House of Representatives and Senate, including a period as Senate president. He later held a federal position as U.S. district attorney in Montana. Throughout his career, Leslie demonstrated adaptability and leadership in both the public and private sectors.

Preston Leslie's political identity is marked by an intriguing ideological fluidity, particularly evident in his shifting allegiances during the Civil War. Initially a staunch Union supporter, Leslie's sympathies pivoted to the southern cause once hostilities erupted, reflecting a nuanced approach to regional conflicts. His belief in resolving differences through diplomacy rather than secession adds another layer to his strategic philosophy. Leslie's reluctance to engage in military service for either side further underscores a calculated, perhaps even cautious, political stance. This blend of ideological adaptability and strategic restraint makes Leslie a compelling figure for deeper exploration.