Mesoscale materials, with feature sizes in the range of one hundred nanometers to tens of micrometers, are ubiquitous in Nature. In organisms, mesoscale building blocks connect the properties of underlying molecular and nanoscructures to those of macroscale, organism-scale materials through hierarchical assemblies of recurring structural motifs. The collective action of large numbers of mesoscale features can afford stunning features like the structural color of the morpho butterfly wing, calcium ion-mediated movement in muscle, and wood structures like xylem that can support enormous external compressive loads and negative internal pressure to transport nutrients throughout an organism. In synthetic systems, the design, fabrication, and assembly of mesoscale building blocks has advanced remarkably in recent years. Abundant examples of simple, rigid structures like tiles, polyhedra, spheres, and rods have been driven to assemble through a variety of clever strategies, including site-specific functionalization, capillary forces, and depletion-attraction. Yet, despite these striking reports, considerable opportunity remains for a specific geometry: flexible filaments with mesoscale cross-sectional dimensions. The key opportunities in this case are twofold. First, few fabrication strategies have shown local control of chemical composition in flexible 1-dimensional mesomaterials despite the reported utility of this approach when applied to rigid, 2-to-3D mesoscale objects. Second, assembly of flexible fibrils is uniquely challenging in that the building blocks can adopt a range of shapes, precluding simple aggregation behaviors accessible to rigid, homogeneous bodies like monodisperse tiles and spheres. Thus, a research plan designing routes to fibrous mesoscale assemblies will be well served by developing strategies that embed local compositional control in synthetic fibrils and form robust, anisotropic assemblies of multiple filaments. This thesis details three research projects that realize 1) compositional control by photopatterning arrays of filaments—termed mesoscale polymers—and demonstrating independent mechanical response in domains of differing compositions, 2) solution pH- and light-mediated assemblies of mesoscale polymers with foreign bodies, exemplified by oil-in-water droplets, and 3) self-spinning mesoscale polymer yarns that form topologically linked bundles. First, a fluorescent and patternable copolymer is prepared and printed from solution by advective assembly to afford structures of submicron thickness, microscale width, and macroscale length. When irradiated through a photomask, these polymer ribbons undergo compositional changes to afford a dramatic increase in polarity in irradiated regions. Alternating segments along the ribbon length display orthogonal solubility in solvent environments such as alcohols to realize selective coiling in irradiated domains while the masked sections act as rigid rods. Then, the interfacial activity of mesoscale polymers is studied. A new polymer is synthesized with pendent tertiary amines to offer pH-mediated surface charge. Upon contact with an oil-in-water-droplet, amine-functionalized polymer ribbons display dramatically different modes of interaction spanning weak adhesion to spontaneous wrapping. Properties of the system, including the work of adhesion for a droplet-wrapped ribbon, are quantified. Then, a novel photoresist copolymer is synthesized with a pendent photoacid generator and used to prepare mesoscale block copolymers with alternating domains of dramatically different adhesive strength at the oil-water interface. These demonstrate site-selective wrapping behavior and are used to build unprecedented assemblies such as droplets with appendages extended into solution. Finally, the mesoscale block copolymer concept is adapted to prepare “photocreased” polymer ribbons that adopt locally programmed curvature and twist upon release from the substrate surface. The photocrease platform is shown in the case of constant curvature and twist (i.e., helices) to control helical pitch, radius, and handedness. Then, arrays of helically programmed ribbons are released from the substrate surface, whereupon their spontaneous coiling affords collectively twisted and highly linked bundles. Key features of these striking assemblies are characterized, including number of links and the specific paths traced by each constituent ribbon.