The thermal decomposition synthesis of long copper nanowires (CuNWs) was achieved by controlling the synthesis parameters. A detailed study was performed to determine the effect of the molar ratio of copper chloride to nickel acetylacetonate, temperature, and stirring rate on the final shape of the products. Transparent electrodes (TEs) were fabricated by wet treatment with acetic acid (AA), without using a sintering process. The low oxidation stability and high surface roughness are the main disadvantages of the CuNW TEs, which limit their applications. In order to overcome these issues, we prepared CuNW/polymer composite TEs by partial embedding of the CuNWs into poly(methyl methacrylate) (PMMA) on poly(ethylene terephthalate) (PET) substrates. The CuNW/PMMA composite TEs exhibit excellent optoelectronic performance (91.3% at 100.7 Ω/sq), low surface roughness (4.6 nm in height), and good mechanical and chemical stability as compared with CuNW TEs. On the basis of these properties, we believe that CuNW-based composite TEs could serve as low-cost materials for a wide range of new optoelectronic devices.