De Ligt has signed a five-year contract, and Mazraoui a four-year deal, both with the option to extend for a further year.
Mazraoui could be immediately called upon by United to deputise at left-back for the injured Luke Shaw – who is expected to return after the first international break – when they host Fulham in the Premier League on Friday (20:00 BST).
United have agreed to pay £38.5m plus £4.3m in add-ons for 25-year-old centre-back De Ligt, who was captain of Erik ten Hag’s Ajax side that reached the 2019 Champions League semi-finals.
Mazraoui also worked with United manager Ten Hag at Ajax, and both players credited his impact on their development.
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“Erik ten Hag shaped the early stages of my career, so he knows how to get the best out of me and I cannot wait to work with him again,” said De Ligt.
“I know what it takes to succeed at the highest level, and I’m determined to continue that record at this special club.”
Mazraoui added: “It is exciting to be reuniting with him [Ten Hag] as I enter the prime years of my career.
“I know what he expects from his players, and I will give everything to help the group be successful.”
