Mercury is a highly toxic environmental pollutant; thus, there is an urgent need to develop new materials for its simultaneous detection and removal from water. In the present study, new oxidized cellulose-based materials, including their Schiff bases, were synthesized and investigated as a sensor–adsorbent for simple, rapid, highly selective, and simultaneous detection and removal of mercury Hg­(II) ions. Cellulose was extracted from the pine needles, etherified, oxidized, and modified to Schiff base by reaction with l-lysine. The well-characterized cellulose Schiff base materials were used as a sensor–adsorbent for Hg­(II) from aqueous solution. Hg­(II) sensing was analysed with naked-eye detection and fluorescence spectroscopy. Schiff base having a decyl chain, C10–O–cell–HCN–Lys, was observed to be an efficient adsorbent with a very high maximum adsorption capacity of 258.75 mg g–1. The data were analyzed on the basis of various kinetic and isotherm models, and pseudo-second-order kinetics and Langmuir isotherm were followed for Hg­(II) adsorption.