We carried out a detailed investigation of Lthium and CNO abundances, including carbon isotope ratios, in RS CVn stars to assess the role of magnetic activity in the mixing of stellar atmospheres. High-resolution spectra were observed at the Moletai Astronomical Observatory to determine the lithium abundances by spectral synthesis of the 6707{AA} line and the CNO abundances using the C2 band heads at 5135 and 5635.5{AA}, CN bands at 6470-6490{AA} and 7980-8005{AA}, and the [O I] line at 6300{AA}. By fitting the ^13^CN band at 8004.7{AA}, we determined the carbon isotope ratios. We determined the main atmospheric parameters and investigated the chemical composition for 32 RS CVn stars. Lithium NLTE abundances were determined for 13 additional stars using archival spectra. We found that iota Gem and HD 179094 have carbon isotope ratios already affected by extra-mixing, even though being in the evolutionary stage below the red giant branch luminosity bump. About half of the low-mass giants, for which the lithium abundance was determined, follow the first dredge-up predictions; however, other stars have Li abundances reduced as predicted by the thermohaline-induced mixing. The intermediate-mass stars have Li abundances reduced as predicted by the rotation-induced mixing. We found that in low-mass chromospherically active RS CVn stars extra mixing of lithium along with carbon isotopes may begin earlier than in normal giants. The Li-rich RS CVn giant V* OP And has quite large C/N and carbon isotope ratios, and raises questions about the origin of its lithium enhancement.