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Monolayer transition-metal dichalcogenides (TMDs) offer a novel platform of optoelectronic devices through their unique excitonic properties. Here, we perform optical transient absorption spectroscopy on monolayer tungsten disulfide (WS₂) under an intense mid-infrared field, and observe a significant (more than a hundred meV) transient tuning of excitonic absorption. Experimentally, mid-infrared pulses (4 um, 100 fs, 0.2 V/nm) induce virtual 1s-np intraexcitonic transitions, and the resulting light-dressing effect is manifested in the optical absorption spectra (500-900 nm), featuring an AC-Stark shift and truncated free-induction decay. Theoretical insights are obtained by analyzing a few-level system within Floquet theory, and also by numerically solving the time-dependent Schrödinger equation for the driven system. Our results demonstrate a new scheme for the ultrafast and enhanced control of the excitonic properties in monolayer TMDs.