The three-star product joins the Terps from nearby St. John's College High School.
Despite going undrafted, Leake should have a good shot to get playing time.
Ellis racked up 52 tackles and 22 pass deflections during his four years in College Park.
"Javon is a big time playmaker who has the ability to hit the home run."
Four Terps could be taken in the draft's late rounds.
Castleberry didn't receive a carry for Michigan last season.
Coach Mike Locksley has focused on building the program's foundation in his first year.
The high-flying kick returner was also named Big Ten Return Specialist of the Year earlier this week.
Leake and Anthony McFarland combined for 66 percent of the team's rushing yards in 2019.