The near wake dynamics developed behind a horizontal cylinder with wall proximity effects are elucidated from laboratory experiments and Large-Eddy Simulations (LES). Fixed vertical gap to diameter (G/D) ratios of 0.5 and 1.0 were investigated for Reynolds numbers equal to 6,666, 10,000 and 13,333. The LES results agreed well with the experimental measurements for the time-averaged flow quantities and captured the upward flow motion developed over the lower half of the flow depth as a consequence of the near-wall effect. The presence of a narrow gap between the cylinder and the bed, i.e. G/D = 0.5, significantly influenced the dynamics of the vortex generation and shedding which, in consequence, led to an increasingly pronounced asymmetric wake distribution with increasing Reynolds number. In the wider gap case of G/D = 1.0, the wake remained relatively symmetrical, with reduced impact of ground proximity. Kelvin-Helmholtz instabilities developed in the upper and lower shear layers were shown to be decoupled as their instantaneous laminar-to-turbulent transition occurred at different downstream distances at any given time. Spanwise rollers were shown to form with an undulating pattern and presented irregularly located vortex dislocations. Furthermore, a ground-vortex induced during the early stages of the lower roller’s generation in the wake lifted off the ground and merged with the vortices to form a single vortical structure. For G/D = 0.5, a positive upwards force was present, and experimental and LES Strouhal number values ranged between 0.28–0.32, while computed drag coefficient values were lower than those typical for unbounded cylinder flows. As for G/D = 1.0, Strouhal numbers decrease to a 0.26–0.30 range whilst drag coefficient increases, further demonstrating the effects on the cylinder wake structure dynamics due to the proximity to a solid boundary.