We use photometric redshifts derived from new u-band through 4.5 mu m Spitzer IRAC photometry in the 4.8 deg(2) of the XMM-LSS field to construct surface density maps in the redshift range of 0.1-1.5. Our density maps show evidence for large-scale structure in the form of filaments spanning several tens of megaparsecs. Using these maps, we identify 339 overdensities that our simulated light-cone analysis suggests are likely associated with dark matter halos with masses, M-halo, log(M-halo/M-circle dot) > 13.7. From this list of overdensities we recover 43 of 70 known X-ray-detected and spectroscopically confirmed clusters. The missing X-ray clusters are largely at lower redshifts and lower masses than our target log(M-halo/M-circle dot) > 13.7. The bulk of the overdensities are compact, but a quarter show extended morphologies that include likely projection effects, clusters embedded in apparent filaments, and at least one potential cluster merger (at z similar to 1.28). The strongest overdensity in our highest-redshift slice (at z similar to 1.5) shows a compact red galaxy core, potentially implying a massive evolved cluster.