Ultrathin composite films consisting of mixtures of metallic (m-) and semiconducting (s-) single-walled carbon nanotubes (SWNTs) with a conjugated block copolymer are developed from a solution-based process. The electronic properties of the films are precisely controlled from metallic to semiconducting to insulating. The tunability of the electronic composite sheets is mainly attributed to (1) the efficient dispersion of SWNTs with a conjugated block copolymer in solution, (2) the control of the number of nanotubes by centrifugation, and (3) the individually networked deposition of SWNTs embedded in the conjugated block copolymer on the target substrate by spin-coating. A highly reliable field-effect transistor with a networked composite film is realized with a specific range of tube density and a high on/off current ratio of approximately 106 which resulted from the Schottky barriers evolved between the individual m- and s-SWNTs in the network. There is also great freedom when choosing both the gate dielectrics and source-drain electrodes for transistors containing the composite films. Furthermore, the fabricated electronic composites are highly transparent, flexible, and chemically robust and thus, they can be conveniently micropatterned by photolithography, as well as by unconventional transfer printing techniques.