In some variations, the present invention provides an oil-resistant paperboard material coated with hydrophobic nanocellulose. The paperboard material is free of a fluorocarbon coating. In other variations, an oil-resistant paper is coated with hydrophobic nanocellulose and is free of a fluorocarbon coating. The hydrophobic nanocellulose may include lignin-coated cellulose nanofibrils, lignin-coated cellulose nanocrystals, or a combination thereof. A process for producing these materials is also provided, comprising: fractionating a lignocellulosic biomass feedstock in the presence of an acid, a solvent for lignin, and water, to generate cellulose-rich solids and a liquid containing hemicellulose and lignin, wherein a portion of the lignin deposits onto a surface of the cellulose-rich solids, thereby rendering the cellulose-rich solids at least partially hydrophobic; mechanically treating the cellulose-rich solids to form hydrophobic nanocellulose; and coating a paper or paperboard material with the hydrophobic nanocellulose, to generate an oil-resistant paper or paperboard material.