Human systems, including institutional systems and informal social networks, are a major arena of modern life. We argue that distinct forms of pragmatic reasoning or "strategic thinking" are required to exercise agency within such systems. This article explores the development of strategic thinking in a youth activism program in which young people worked for social change. These youth came to understand different human systems, the school board, teachers, and students, and they learned to employ three strategic modes of reasoning: seeking strategic information, framing communications to the audience, and sequential contingency thinking. Although youth described themselves as agents of their development, adults played important roles in supporting their experience of a cycle of experiential learning. These findings suggest how the new cognitive potentials of adolescence allow youth to develop modes of reasoning that expand their capacity to exercise agency over a longer arc of time and across a wider interpersonal space.