Conjugated polymers with both high charge mobilities and responsive functionality have received increasing attention because of their promising applications in organic electronic devices and chemo-/biosensors. We successfully incorporated thymine groups in the side chains of diketopyrrolopyrrole (DPP)-based conjugated polymer PDPP4T-T. Our studies reveal that thin films of PDPP4T-T exhibit improved crystallinity and higher charge mobility compared with analogous polymers with pure alkyl chains. This is attributed to the formation of H-bonding among thymine groups, which could strengthen interchain interactions and thus facilitate an ordered packing of polymer chains. Importantly, thin films of PDPP4T-T containing either Pd(II) or Hg(II) ions were successfully fabricated via air–water interface coordination reactions of thymine groups with ions. Thin-film field-effect transistors (FETs) of PDPP4T-T containing Pd(II) ions exhibit a sensitive and selective response toward CO even at a low concentration of 10 ppb. Such an FET-based CO sensor with high sensitivity and good selectivity is achieved for the first time by using polymeric semiconductors. Alternatively, FETs with PDPP4T-T-Hg(II) thin films can be utilized to detect H₂S with good selectivity and high sensitivity (down to 1 ppb). Therefore, incorporating thymine groups into conjugated polymers can not only improve semiconducting mobilities but also endow semiconducting thin films with sensing functionality.