In need of depth at the position, the Dolphins worked out a pair of free agent corners earlier today. In the case of Mackensie Alexander, that has resulted in a contract (as confirmed, on Twitter, by his agent). 
The 28-year-old was a second-round pick of the Vikings in 2016. He played sparingly during his first two seasons, though he recorded his first career interception in 2017. The following year, he registered his first six NFL starts, and totaled 10 pass deflections and four sacks. The Floridian signed in Cincinnati as a free agent in 2020, where he played the highest snap percentage (75%) of his career.
Alexander returned to Minnesota last season, but had the worst season of his career in terms of pass coverage, surrendering five touchdowns and a passer rating above 119. At the onset of free agency, it became clear that the Vikings would not be re-signing him. After nearly the entire offseason, he has now found a roster spot.
Miami has Xavien Howard, Byron Jones and Nik Needham at the top of their CB depth chart, though Jones remains a question mark from a health perspective to be available by the start of the regular season. The team was very much in need of reinforcements behind that trio, especially after training camp standout Trill Williams suffered a torn ACL during their first preseason game.
Alexander will provide Miami with experienced depth at a minimum, something which was lacking at the No. 4 corner spot before today. He is also capable of stepping into a starting role, however, which could turn this signing into an effective one. Alexander’s arrival could also, notably, put 2020 first-rounder Noah Igbinoghene on the roster bubble, as noted by Barry Jackson of the Miami Herald (Twitter link).
In addition to Alexander, the Dolphins worked out UDFA Raleigh Texada, who had a five-year career at Baylor (Twitter link via Aaron Wilson of Pro Football Network). He likely won’t be needed now, with Alexander on the books.