Magnetic field observations from the JPL vector helium magnetometer on the ISEE-3/ICE spacecraft are used to investigate long term temporal variations in the interplanetary magnetic field during solar cycle 21. As reported by previous studies, IMF intensity exhibited a broad decrease during the last sunspot minimum with average values of only 4.7 nT being measured in mid-1976. It is shown in this study that IMF magnitude peaked in late 1982 at 9.0 nT, or approximately twice its solar minimum intensity. The strength of the interplanetary field decreased rapidly during 1983 and 1984 to a value of 6.2 nT in early 1985. Comparison with the auroral AE index indicates that the post-maximum peak in IMF strength in 1981-83 was associated with a ~50% increase in mean substorm activity levels. This is the first time that a long term variation in the IMF has been observed to produce an increase in geomagnetic activity comparable to, or greater than the enhancements accompanying the well known declining phase high speed streams. A marked decrease in Parker spiral angle, which indicated the presence of high speed streams in 1973-75 during the declining phase of solar cycle 20, is also observed 11 years later in 1984-85. The variations in the intensity of the interplanetary magnetic field, and its components, are shown to be only moderately correlated with the Mt. Wilson magnetograph measurements of full disk magnetic flux, i.e., linear correlation coefficients of 0.5 to 0.7. The implications of these results for the long term evolution of the interplanetary magnetic field and its effects on geomagnetic activity are discussed.