A 3-0 humbling at home to Charlton Athletic on Friday left Wigan six points from safety, a margin that could increase on Saturday depending on other results.
Mackay has already added nine new faces at the DW Stadium since replacing Uwe Rosler in November, with a host of players departing during the transfer window as the Scot looks to revitalise their fortunes.
Pennant becomes his 10th signing after agreeing a contract until the end of the season, the 32-year-old joining on a free transfer after his contract with Pune City came to an end.
"Jermaine needs no introduction in terms of what he has achieved in the game," said Mackay.
"He's got terrific experience and is hungry for a way back into the English scene. There were a number of clubs interested, so we are delighted we can now call upon him to help us.
"He has been with us throughout the week and has trained excellently, he's brought a breath of fresh air to the place and we've been bowled over by his hunger and enthusiasm.
"Jermaine has integrated straight away into the group and I'm sure he's going to be a great addition for us."
Pennant could make his debut on Tuesday for the 2013 FA Cup winners, when they host Cardiff City looking for a first home win since August 30.
