Resonant microstrings show promise as a new analytical tool for thermal characterization of polymers with only few nanograms of sample. The detection of the glass transition temperature (Tg) of an amorphous poly(d,l-lactide) (PDLLA) and a semicrystalline poly(l-lactide) (PLLA) is investigated. The polymers are spray coated on one side of the resonating microstrings. The resonance frequency and quality factor (Q) are measured simultaneously as a function of temperature. Change in the resonance frequency reflects a change in static tensile stress, which yields information about the Young’s modulus of the polymer, and a change in Q reflects the change in damping of the polymer-coated string. The frequency response of the microstring is validated with an analytical model. From the frequency independent tensile stress change, static Tg values of 40.6 and 57.6 °C were measured for PDLLA and PLLA, respectively. The frequency-dependent damping from Q indicates higher Tg values of 62.6 and 88.8 °C for PDLLA and PLLA, respectively, at ∼105 Hz. Resonant microstrings facilitate thermal analysis of nanogram polymer samples measuring the static and a dynamic glass transition temperature simultaneously.