The 34-year-old Zimbabwe-born loosehead prop qualified for South Africa on residency rules and went on to win a record 117 caps for a Springboks forward, culminating in November’s World Cup final win over England in Japan.
Old Glory head coach Andrew Douglas said: “It’s exciting to have one of the world’s best players here at Old Glory.
“As a coach, not only am I excited about what Tendai brings to the pitch, but also what he brings to our environment. To have a guy who has played 117 Test matches yet is still so hard-working and humble can only be beneficial for us. The signing takes the league to another level.”
Mtawarira, who will join from Super Rugby side the Sharks for Old Glory’s inaugural season in a competition expanding to 12 franchises next year said: “I really want to thank the whole Old Glory organisation for making this happen.
“I can’t wait to make an impact on and off the field and help to contribute to making Old Glory the best team in this league.”
The SRU struck a deal to take a minority stake in Old Glory in March which it described as a “strategic partnership”, which the union’s chief operating officer Dominic McKay taking a place on the franchise’s board.
The MLR season runs from February to June and had nine franchises competing in its second season following the arrival of the Toronto Arrows earlier this year.
The number of teams will expand to 12 in 2020 with the inclusion of Old Glory DC, along with teams from Atlanta and New England.