A bicycle operating device mounting assembly is basically provided with a handlebar mounting clamp and an adapter member. The handlebar mounting clamp has a clamp opening. The adapter member includes at least a first part that is detachably disposed within the clamp opening. The first part is configured to be clamped between the handlebar mounting clamp and a handlebar as the handlebar mounting clamp is tightened in a state that the first part is disposed within the clamp opening.