The MMPI and MMPI-2 validity scales have long been accepted as standard tools in the assessment of feigned mental disorders (FMD) based on their extensive empirical validation. Studies are now examining MMPI-2-RF with modified validity scales plus the new Infrequent Somatic Responses Scale (FS) and the recently-adapted Response Bias Scale (RBS). The current investigation used a known-groups design to examine the effectiveness of the MMPI-2-RF for differentiating FMD and feigned cognitive impairment (FCI) from patients with genuine disorders for a large civil forensic sample. Criterion measures included the Structured Interview of Reported Symptoms-2 (SIRS-2) for the FMD group, and below-chance performances on the Victoria Symptom Validity Test (VSVT) and the Test of Memory Malingering (TOMM) for the FCI group. For FMD, both F-r and FP-r produced very large effect sizes (ds > 2.00). Moreover, the absence of severe elevations (≥80 T) on F-r proved effective at ruling-out most FMD. For the current study, a FP-r cut score ≥90 T for FMD produced virtually no false-positives (0.01) and only a moderate level of false-alarms. As predicted by its detection strategies, most MMPI-2-RF validity scales have limited effectiveness with the FCI group. However, FBS-r and RBS may be useful in conjunction with other clinical data for ruling out FCI for genuine neuropsychological consults. An entirely separate concern is whether certain diagnostic groups, such as major depression, will have marked elevations on MMPI-2-RF scales thereby increasing the likelihood of false-positives. On this point, FP-r performed exceptionally well with unelevated scores (Ms < 55 T) consistently across diagnostic categories.