Abstract: A bicycle-mountable carrier assembly includes a mounting assembly attachable to a rail of the bicycle's saddle. At least one payload support is joined, e.g., pivotably, to the mounting assembly. The payload support defines a structure for mounting at least one payload carrier attachment in a secure but removable fashion. One or more payload supports may be provided, and one or more mounting structures may be provided on each payload support. The mounting structures may be provided in a rear-and side-facing positions and in different angular orientations, for carrying of payloads in a variety of different positions and orientations. The carrier further includes at least one payload carrier attachment defining a mounting structure complementary to the payload support's mounting structure. The payload carrier may be adapted to function as a fishing rod holder, a fishing rod case holder, a water bottle, a fishing tackle box, or another payload as desired.