The day-of-year (DOY) variation in the average value of a solar wind driver of geomagnetic activity has been shown to explain only a minority of the observed amplitude of DOY variation in geomagnetic activity. The proxies for solar wind driving used to show this are averages of a solar wind measurement in the same hour or 3-hour interval as the geomagnetic activity measurement. This model of solar wind driving of geomagnetic activity does not account for the observation that the solar wind state in a given time interval can have an effect on activity that lasts many hours. In this work we show a model that includes the solar wind time history predicts a much higher DOY variation in both auroral zone and midlatitude geomagnetic activity. This model is used to estimate the solar wind contribution in two geomagnetic activity measures that exhibit a semiannual DOY variation: the am index and postmidnight ground magnetic field measurements in the auroral zone. The estimated solar wind driver contribution to the DOY variation in the am index is 75%, which is approximately twice the amount of previous estimates. Solar wind driving is estimated to explain 40–60% of the DOY variation in the magnetic field measured by auroral zone ground magnetometers in the postmidnight sector, where previous estimates were near zero.