The Republic of Ireland centre-half has joined on a temporary basis until May 4 as the London club look to shore up their backline for the season run-in
St Ledger, who has signed until May 4, will however be prohibited from making contact with the Lions until Friday night's clash between the two clubs passes.
Millwall are currently short of options at the back with Karleigh Osborne sidelined with a hamstring injury and manager Kenny Jackett will hope the addition of the Republic of Ireland international will help bolster their backline and stave off the threat of relegation.
St Ledger will compete with Danny Shittu and Mark Beevers for a starting place in a team that currently sits 14th and just five points above the dropzone.
The 28-year-old has also been granted permission by Leicester to play in the FA Cup for the Lions as they take on Wigan in the semi-finals at Wembley in April.