Gold nanoparticles supported on a quartz substrate are fabricated by e-beam lithography and subjected to nanosecond laser irradiation to induce coalescence. In combination with ex-situ scanning electron microscopy and optical extinction spectra, transmission is monitored in-situ to determine the characteristic coalescence time, the threshold fluence for the onset of coalescence and the produced particle morphology. Parallel molecular dynamics simulation is employed and shown to reasonably predict the coalescence time, thus explaining the experimental results.