Volunteer programs can benefit from a deeper understanding of the motivations and experiences of people engaged in citizen science. Research to date has studied motivations of citizen scientists and tree-planting volunteers. Less work has focused on tree-monitoring volunteers, a role that is rapidly increasing as more cities involve the public in monitoring the urban forest. Researchers conducted an assessment of volunteers (n = 636 respondents) of the TreesCount! 2015 street tree census in New York City, New York, U.S., to understand volunteers' demographics, motivations, experiences, and levels of civic engagement. Semistructured interviews (n = 40) were also conducted on a subset of the initial assessment respondents, to deepen understanding of these factors. Like tree-planting volunteers in previous studies, volunteers were more likely to be highly educated, female, white, and with high income levels. Top self-identified motivations for participation included personal values, wanting to contribute, and a desire for education or learning. Demographics correlated with different motivations, suggesting opportunities for targeting recruitment efforts to better reach underrepresented populations. Researchers also found motivations shifted slightly in post-census interviews, also identifying a new theme of exploring the city. Street-tree monitoring presents opportunities for contributing to one's community or city, and for learning about trees and urban nature, suggesting these acts of engagement can both strengthen connections to social-ecological systems and provide personal benefits. At the same time, considering volunteer motivations, experiences, and outcomes when designing programs can positively affect participation turnout, effort, and retention.