We characterize the Spitzer Wide-area Infrared Extragalactic Legacy Survey (SWIRE) galaxy populations in the SWIRE validation field within the Lockman Hole, based on the 3.6-24 μm Spitzer data and deep U, g', r', i' optical imaging within an area ~1/3 deg2 for ~16,000 Spitzer SWIRE sources. The entire SWIRE survey will discover over 2.3 million galaxies at 3.6 μm and almost 350,000 at 24 μm ~70,000 of these will be five-band 3.6-24 μm detections. The colors cover a broad range, generally well represented by redshifted spectral energy distributions of known galaxy populations; however, significant samples of unusually blue objects in the [3.6]-[4.5] color are found, as well as many objects very red in the 3.6-24 μm mid-IR. Nine of these are investigated and are interpreted as star-forming systems, starbursts, and active galactic nuclei (AGNs) from z=0.37 to 2.8, with luminosities from LIR=1010.3 to 1013.7 Lsolar.