The 30-year-old Russian, who missed out on the season finale last year after qualifying every year from 2019 to 2024, joined Alexander Zverev, Carlos Alcaraz and Ben Shelton in the eight-man field.
Jannik Sinner has also qualified but will not compete due to injury, leaving four places still available.
Medvedev has enjoyed a resurgent season in which he has surpassed the 50-win mark for the first time since 2023 and claimed four titles.
Medvedev took control early against Mannarino with a break of serve and eased through the opening set.
The former US Open champion broke again at 3-3 in the second set and closed out the match with another break.
Medvedev will face either Tommy Paul or Alcaraz for a place in the final.
