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In this study, we explore the use of unsteady transit time distribution (TTD) theory to model solute transport in biofilters, a popular form of nature‐based or "green" storm water infrastructure (GSI). TTD theory has the potential to address many unresolved challenges associated with predicting pollutant fate and transport through these systems, including unsteadiness in the water balance (time‐varying inflows, outflows, and storage), unsteadiness in pollutant loading, time‐dependent reactions, and scale‐up to GSI networks and urban catchments. From a solution to the unsteady age conservation equation under uniform sampling, we derive an explicit expression for solute breakthrough during and after one or more storm events. The solution is calibrated and validated with breakthrough data from 17 simulated storms at a field‐scale biofilter test facility in Southern California, using bromide as a conservative tracer. TTD theory closely reproduces bromide breakthrough concentrations, provided that lateral exchange with the surrounding soil is accounted for. At any given time, according to theory, more than half of the water in storage is from the most recent storm, while the rest is a mixture of penultimate and earlier storms. Thus, key management endpoints, such as the pollutant treatment credit attributable to GSI, are likely to depend on the evolving age distribution of water stored and released by these systems. Plain Language Summary: Conventional drainage systems are designed to move storm water as quickly as possible away from cities. By contrast, green storm water infrastructure (GSI) captures and retains storm water as close as possible to where the rain falls. As storm water runoff is a leading cause of nonpoint source pollution, quantifying the pollutant removal services provided by GSI is a top priority. In this paper we propose and test a mathematical framework—unsteady transit time distribution (TTD) theory—for modeling and predicting solute transport in biofilters, a popular form of GSI. From field data collected at a biofilter test facility in Southern California, we demonstrate that TTD theory closely tracks measured solute transport through these systems during and after storms. The theory's simplicity and predictive power make it ideally suited to model solute fate and transport at the scale of individual biofilters, as well as GSI networks and the urban catchments in which they are embedded. Key Points: A solution is derived from unsteady transit time distribution theory to model solute transport through green storm water infrastructureThe solution is calibrated and validated with data from 17 simulated storm events at a field‐scale test facility in Southern CaliforniaThe solution reproduces measured breakthrough concentrations, provided lateral exchange with the surrounding soil is taken into account [ABSTRACT FROM AUTHOR]