Icardi scored in injury time for the Ligue 1 leaders with a header from Achraf Hakimi’s dinked cross from the byline – minutes after Sergio Ramos had been sent off for two bookings.
Lorient, 19th in the table, scored five minutes before the break when Thomas Monconduit’s shot went in off the bar.
PSG’s Angel di Maria had a late chance to win it but was denied by Paul Nardi.
Di Maria had earlier wasted a good opportunity early in the second half, shooting over from eight yards. Lionel Messi also had an effort saved by Nardi with the Argentina forward curling another attempt just wide.
Defender Ramos, in only his second game for PSG after joining on a free transfer in the summer following the end of his Real Madrid contract, received the 27th red card of his career.
He came on as substitute at the start of the second half, but was booked for a high boot in the 81st minute and then picked up his second yellow card five minutes later after cynically blocking Terem Moffi as he looked to sprint towards goal.
This was only the fifth time in 19 league games this season that Mauricio Pochettino’s PSG had failed to win and they sit 13 points clear of second-placed Nice.