Fingolimod is a sphingosine 1-phosphate-receptor modulator approved for the oral treatment of relapsing–remitting multiple sclerosis (RRMS), a form of MS characterized by a pattern of exacerbation of neurological symptoms followed by recovery. Here, we validated a simple and rapid liquid chromatography–tandem mass spectrometry method for the measurement of the concentrations of Fingolimod and its active metabolite Fingolimod-Phosphate (Fingolimod-P) in human plasma. The lower limits of quantification were set at 0.3 and 1.5 ng/mL for Fingolimod and Fingolimod-P, respectively, and the linearity was in the range 0.3–150 ng Fingolimod/mL and 1.5–150 ng Fingolimod-P/mL. After protein precipitation, the extraction recoveries of both analytes were always above 60% with minimal matrix effect. The method was accurate and precise, satisfying the criteria set in the European Medicine Agency guidelines for bioanalytical method validation. The method was then applied to measure Fingolimod and Fingolimod-P concentrations in the plasma of 15 RRMS patients under chronic treatment with Fingolimod, administered daily at the dose of 0.5 mg for up to 24 months. No significant differences were observed between samples collected at 6, 12 and 24 months for both analytes, indicating that the drug’s bioavailability was unaffected by multiple daily doses up to 24 months. The levels of Fingolimod-P were about two-fold higher than the levels of the parent compound. The availability of this analytical method can allow the monitoring of the impact of plasma levels of the drug and its metabolite on inter-individual variability in clinical responses.