A thumb assembly mountable to a stick of an excavator is provided. The thumb assembly comprises a base member having at least a first joint location, a thumb member pivotally connected to the base member, and first and second linkage members pivotally connected at their first ends to the thumb member and the joint location respectively. The linkage members are connected by a knee joint at their second ends such that they are foldable relative to each other between a working position and a storage position in which the thumb member extends close to the base member. The knee joint permits both relative rotation and relative sliding of the first and second linkage members so that the thumb assembly can be moved between the working position and the storage position without disconnecting the linkage members from the thumb member and the joint location on the base member.