Ultrathin nanocomposite films fabricated by layer-by-layer (LbL) assembly are an emerging class of coatings with remarkable barrier performance. Here, we examine LBL polymer-clay nanocomposite (PCN) coatings that afford anti-corrosion properties comparable to those exhibited by more traditional films that are orders of magnitude thicker. PCN barrier thin films comprised of up to 40 bilayers of polyethyleneimine (PEI) and exfoliated laponite (LAP), montmorillonite (MMT), or vermiculite (VMT) clay platelets were coated onto steel plates using a scalable LBL dip-coating process. A combination of electrochemical impedance spectroscopy and ex-situ electron and vibrational spectroscopy were used to evaluate coating performance during exposure in 0.6 M NaCl for up to 7 days. A PEI/VMT crosslinked coating of only 250 nm thick slowed the corrosion rate of mild steel by three orders of magnitude (Rct = 106 vs. 103 -cm2) after seven days in 0.6 M NaCl. This performance exceeded the reported performance of other LBL coatings along with much thicker multilayer composite coatings and several conventional pretreatments on mild steel under similar test conditions. The exceptional performance is attributed to the effective barrier properties imparted by a high loading (>80 wt%) of orientated, high aspect ratio (2000:1) platelet layers in a nanobrick wall morphology. Substitution of VMT with lower aspect ratio clay platelets of either MMT (400:1) or LAP (30:1) showed that coated steel corrosion rates and apparent coating permeability scales with platelet size following LAP > MMT >> VMT. Thus, this study has for the first time shown that platelet aspect ratio is a primary means of improving the performance LbL nanocomposite coatings which is in line with the expected behavior of conventional nanocomposites. Given film debonding from the substrate was observed as a common failure mode during exposure for all coating types, replacement of the PEI polymer component with a more debonding-resistant polymer may be an additional pathway to further improve LbL PCN anticorrosion performance.