This report describes the synthesis of miktoarm brush-arm star polymers (BASPs) from branched and linear norbornene-terminated macromonomers (MMs) via the brush-first ring-opening metathesis polymerization (ROMP) method. First, a polystyrene (PS)-branch-poly(lactic acid) (PLA) MM is synthesized via a combination of atom transfer radical polymerization (ATRP), tin(II)-mediated ring opening polymerization, and copper-catalyzed azide-alkyne cycloaddition reactions. Graft-through ROMP of this MM followed by in situ cross-linking with a photo-cleavable bis-norbornene derivative provided nanoscopic BASPs with photodegradable cores and a precise 1:1 PS:PLA arm composition. Three-miktoarm BASPs are prepared in an analogous manner via copolymerization of the same PS-branch-PLA MM with a poly(ethylene glycol) (PEG) MM prior to cross-linking. Intramolecular phase segregation of these miktoarm BASPs is characterized by transmission electron microscopy (TEM); a UV-induced structural rearrangement from three-faced Janus particles to micelles is observed.