His victory comes two weeks after Hagestad, a 32-year-old from California, went 2-1 in helping the Americans rally to win the Walker Cup at St. Andrews.
Hagestad earns a third trip to the Masters and a spot in the U.S. Open next year at Pinehurst No. 2. He was the first U.S. Mid-Amateur champion to make the cut at the Masters in 2017 and was low amateur for the week.
In the U.S. Women's Mid-Amateur at Stonewall Golf Club in Elverson, Pennsylvania, Kimberly Dingh of Midland, Michigan, won five of the last six holes to rally for a 2-up victory over Kelsey Chugg in the 18-hole championship match.
Dingh earns a spot in the U.S. Women's Open next year.
Hagestad had a 5-up lead after 18 holes on Thursday, and the rest of the 36-hole championship match concluded Friday morning because of fog delays that disrupted the schedule.
In chilly weather, Hagestad had two quick birdies and saved par to go 7 up. Beck, who played college golf at Wake Forest, twice managed to cut the deficit to 3 up until he ran out of holes and Hagestad clinched it with a par on the 16th hole.
The Mid-Amateur is for players 25 years or older.