Maharaj will replace New Zealand captain Mitchell Santner, who has been forced to withdraw from the tournament due to a shoulder injury.
The 36-year-old Maharaj was part of the Proteas squad at the T20 World Cup earlier this year and has 54 caps in the shortest format at international level.
The left-arm spinner captained Pretoria Capitals in the fourth season of the SA20, finishing the campaign with 12 wickets in as many games as his side lost in the final against Sunrisers Eastern Cape.
Mumbai Indians' spin options with the addition of Maharaj also includes Afghanitsan's AM Ghazanfar, Mayank Markande and Raghu Sharma as well as England allrounder Will Jacks.
Mumbai Indians have won just two of their seven games so far this season and are ninth on the points table ahead of their next clash against Sunrisers Hyderabad at the Wankhede Stadium on Wednesday.
Josh Hazlewood was the star bowler on show to help the Royal Challengers Bangalore thrash Delhi Capitals by nine wickets in their Indian Premier League clash at the Arun Jaitley Stadium.
Delhi Capitals fast bowler Lungi Ngidi has been discharged from hospital and is in a stable condition after being stretchered off during Saturday's IPL clash against Punjab Kings at the Arun Jaitley Stadium.
It was record-breaking day at the IPL as Punjab Kings produced the highest-ever run-chase to beat Delhi Capitals while Sunrisers Hyderabad overcame Rajasthan Royals.
Virat Kohli became only the third player in IPL history to surpass 300 sixes, reaching the milestone during a match-winning performance for Royal Challengers Bengaluru against Gujarat Titans on Friday.
