Manganese dioxide (MnO2) is an interesting material due to its excellent biocompatibility and magnetic properties. Adsorption of DNA to MnO2 is potentially of interest for drug delivery and sensing applications. However, little fundamental understanding is known about their interactions. In this work, carboxyfluorescein (FAM)-labeled DNA oligonucleotides were used to explore the effect of salt concentration, pH, and DNA sequence and length for adsorption by MnO2, and comparisons were made with graphene oxide (GO). The DNA desorbs from MnO2 by free inorganic phosphate, while it desorbs from GO by adenosine and urea. Therefore, DNA is mainly adsorbed on MnO2 through its phosphate backbone, and DNA has a stronger affinity on MnO2 than on GO based on a salt-shock assay. At the same time, DNA was used to study the effect of thiol containing compounds on the dissolution of MnO2. Adsorbed DNA was released from MnO2 after its dissolution by thiol, but not from other metal oxides with lower solubility such as CeO2, TiO2, and Fe3O4. DNA-functionalized MnO2 was then used for detecting glutathione (GSH) with a detection limit of 383 nM. Finally, DNA was found to inhibit the peroxidase-like activity of MnO2. This study has offered many fundamental insights into the interaction between MnO2 and two important biomolecules: DNA and thiol-containing compounds.