Visual input to the brain is highly dynamic during natural behavior due to movements of the eyes, head, and body through complex environments. Previous studies have shown that neurons in mouse primary visual cortex (V1) respond to eye and head movements, but how these signals are integrated with visual processing during free movement is unknown since visual physiology is generally performed under conditions of head-fixation. To address this, we performed the first measurements of visual receptive fields (RFs) from freely moving animals by using single-unit electrophysiology in V1 while simultaneously measuring the mouse’s visual scene and eye/head position using head-mounted cameras and an inertial measurement unit (IMU). These RFs, estimated using a generalized linear model (GLM), explained a significant amount of neural activity and closely matched RFs measured in the same cells under head-fixed conditions. Many V1 neurons also showed significant tuning for eye position and head orientation during free movement. By incorporating these variables into the GLM, we found that this tuning was best explained by different combinations of multiplicative and additive integration of visual scene and eye/head position across the population, consistent with theoretical models of gain fields and linear/nonlinear mixed selectivity. Our results provide new insight into coding in mouse V1, with implications for visual processing beyond traditional head-fixed preparations. More generally, this work provides a paradigm and requisite tools for performing visual physiology under natural conditions, including active sensing and ethological behavior.