After emerging from Southend United’s youth academy, 21-year-old Bishop made his League debut for the Essex club, aged just 18 in December 2017.
Stags boss Nigel Clough said: “Nathan is a promising young goalkeeper with good experience in League One with Southend.
“For a young goalkeeper, to get those 30 games in League One was great for him.
“He showed a degree of maturity in that time, which will have hopefully improved further in the past 12 months.
“Although he hasn’t played too much in the past year, he has trained with Manchester United’s first team every day, which is the next best thing to playing matches.
“You cannot fail to improve in that environment with United’s coaches and playing staff.
“He is the best we can get in this position.”
The Londoner has provided excellent support for his fellow goalkeepers throughout the last two seasons at Old Trafford.
Bishop will remain with United for the beginning of pre-season before joining up with the Stags at an agreed time ahead of the Sky Bet League Two campaign.