A new analytical method was developed for the routine Therapeutic Drug Monitoring of 8 antifungals compounds in 50 μL of plasma: isavuconazole (ISZ), voriconazole (VRZ), posaconazole (PSZ), fluconazole (FCZ), caspofungin (CSF), flucytosine (5FC), itraconazole (ITZ) and its metabolite OH-itraconazole (OH-ITZ). After adding 50 μL of the internal standard, which consisted in a mixture of the deuterated isotopes of the quantified compounds, the sample treatment consisted in a simple protein precipitation with 400 μL of acetonitrile. Five microliters of the supernatant were directly injected into the chromatographic system. The chromatographic separation was performed with a Waters C18-BEH column and a mobile phase consisting in a mixture of water and acetonitrile, both containing 0.1% of formic acid. The total run time was 3 min and the detection of the analytes was performed by electrospray ionization in a positive mode using selected reaction monitoring. Intra and inter-day precision and inaccuracy were < 15% over the calibration ranges that were determined according to their clinical relevance: 0.20–20.0 mg/L for ISZ, VRZ, PSZ, ITZ, and OH-ITZ; 0.50–50.0 mg/L for FCZ and CSF; 2.00–200 mg/L for 5FC. This simple and fast method was found suitable for routine therapeutic drug monitoring.