"The deal is done and he'll have a medical on Monday," Ferguson told the club's official website on Friday.
Although no financial details were revealed, the fee is reportedly to be in the region of 5.8 million Euros ($7 million).
The Scottish tactician believes the French international will be a perfect replacement for injured left back Gabriel Heinze.
"We've known about him for quite a while and he's improved every year. He's been playing at left-back for the last three years at Monaco and we've been watching him all season particularly since Heinze got his injury," he said.
"Patrice is naturally left-footed and very quick. His age is right and he gives us a different option at left-back," he added.
Evra could make his debut with the Red Devils on Wednesday when Blackburn Rovers host United in the Carling Cup semi-final first leg.
The 24-year-old was aiming to make a switch to the Old Trafford after he fell out with Monaco board and was pulled out of the squad to face Auxerre on Thursday.
Evra is United's second signing in the January transfer window after luring Serbia and Montenegro international Nemanja Vidic from Spartak Moscow.
United are in second place in the Premiership table with 45 points, 13 points behind leaders Chelsea.