After already triggering a one-year extension, 24-year-old Stewart has committed to a further two years, keeping him at Adams Park until the summer of 2020.
Williams, 19, moves to the League Two club on a five-month loan.
He has appeared for the Chairboys in two pre-season friendlies.
Wycombe finished ninth last season and start the new campaign with a home game against Lincoln City on Saturday.
Find all the latest football transfers on our dedicated page.
Wycombe have signed QPR full-back Jack Williams on loan, while fellow defender Anthony Stewart has signed a new contract with the League Two club.