The invention relates to a mounting system for an architectural covering comprising a roller and a bearing for rotatably mounting the roller to a holding member. The bearing comprises a length variation member which is biased to be in a first state yielding a bearing length which allows the roller to be held by the holding member, and which can be manipulated into a second state, in which the bearing length is reduced such that the roller is removable from the holding member. The length variation member comprises at least one sloped portion that extends along at least part of its outer circumference and in the first state extends largely or completely outside the roller. The sloped portion is dimensioned such that the length variation member can be urged to the second state by applying a force on the sloped portion in a single direction.