Discovery and understanding of mechanisms for kinetically controlled growth of metal nanoclusters can be enabled by realistic atomistic-level modeling with ab initio kinetics. KMC simulation of such a model for Au deposition on Ag(100) films reveals the formation of single-atom-wide Au chains below 275 K, even though 2D islands are thermodynamically preferred. Chain formation is shown to reflect a combination of strong linear trio attractions guiding assembly and a weak driving force and slow rate of transformation of 1D chains to 2D islands (or sometimes irreversible rounding of adatoms from chain sides to ends). Behavior can also be tuned by quantum size effects: chain formation predominates on 2-monolayer Ag(100) films supported on NiAl(100) at 250 K for low coverages but not on 1- or 3-monolayer films, and longer chains form than on bulk Ag(100). Our predictive kinetic modeling shows the potential for simulation-guided discovery and analysis of novel self-assembly processes.