We have studied electrochemically controlled molecular adsorption onto metallic nanowires with quantized conductance. Each nanowire is created mechanically by separating two Au electrodes in contact, and is atomically thin when its conductance is reduced to the lowest few quantum steps. The presence of molecular adsorbates greatly increases the probability of forming a nanowire with conductance at a fractional quantum step. In addition the average length that a nanowire can be stretched before its conductance jumps to a lower step increases by as many as three to four times. The degree of the adsorbate effects correlates with the binding strengths of the three molecules studied, 2,2'-bipyridine (22BPY), adenine and mercaptopropionic acid (MPA), to Au electrodes. Our study shows that molecular adsorbates can affect the atomic configuration and the stability of atomically thin metallic wires.