A modified Trichoderma reesei Family 6 (TrCel6A) cellulase enzyme comprising amino acid substitutions at one or more positions selected from the group consisting of 129, 322, 363 and 410 of SEQ ID NO: 1 is provided. Genetic constructs and genetically modified microbes comprising nucleic sequences encoding the modified TrCel6a cellulase are also provided. The modified TrCel6A cellulase of the invention display at least a 15% decrease in inactivation by lignin relative to a parental TrCel6A cellulase from which the modified TrCel6A is derived. Such cellulases find use in a variety of applications in industry requiring enzymatic hydrolysis of cellulose in the presence of lignin, e.g., the hydrolysis of pretreated lignocellulosic feedstocks for the production of fermentable sugars, sugar alcohols and fuel alcohols.