[{"_id":"Document 1","data":{"list_of_data":[{"key":"KEY_1","value":"value A","valueId":"1234"},{"key":"KEY_2","value":"value B","valueId":"1235"},{"key":"KEY_3","value":"value C","valueId":"1236"}]}},{"_id":"Document 2","data":{"list_of_data":[{"key":"KEY_1","value":"value B","valueId":"1236"},{"key":"KEY_2","value":"value A","valueId":"1237"},{"key":"KEY_3","value":"value C","valueId":"1238"}]}}]

db.collection.aggregate([{"$set":{"sortVal":{"$reduce":{"input":"$data.list_of_data","initialValue":"","in":{"$cond":[{"$eq":["$$this.key","KEY_1"/** sort key*/]},"$$this.value","$$value"]}}}}},{"$sort":{"sortVal":1}},{"$unset":"sortVal"}])