The frontman notched 20 goals in 2020/21 for the O’s, including a run of five goals in the first five matches of the season.
Stags’ boss Nigel Clough said: “We needed a goalscorer and that’s where Danny fits the bill.“We were impressed with the variety of goals he scored last season, with not too many situations and chances created for him.
“He scored 20 goals last season and I’m hoping that with the way we play, and the number of chances we create for him, he’ll get even more.
“At 28, I think he’s in his prime in terms of scoring goals. I’m hoping his experience will come together to ensure that he’s a very good signing for us.”
Over his career Johnson has bagged an impressive 165 goals in 322 appearances.
Johnson began his career as a youngster with Middlesbrough, before moving into the youth system at Hartlepool.
After spells with Harrogate Town and Billingham Synthonia, the now-28-year-old would enjoy his most prolific spell with Guisborough Town.
Johnson netted an astonishing 59 goals in all competitions in the 2013/14 campaign, leading him to the Manager’s Player of the Year award, the Northern Football League’s Player of the Year award and BBC Newcastle’s Player of the Year award.
Not only this, but his stunning goalscoring exploits landed him a move to Championship side Cardiff City, where he scored regularly for the U21 development squad.
A move to Gateshead followed, with Johnson netting 52 goals in three years with the Tynesiders.
Spells in Scotland with Motherwell and Dundee preceded his switch to East London, netting 20 goals in 48 appearances for Leyton Orient in 2020/21.