Murray fights back in gruelling five-setter
The Scot looked destined for a shock second round defeat in the US Open before recovering to win 5-7 4-6 6-1 6-3 6-1 in humid conditions at Flushing Meadows.
He now face Brazilian 30th seed Thomaz Bellucci of Brazil in the third round.
The last time Murray lost in the first week of a Grand Slam was in 2010, but he struggled to cope with world number 35 Mannarino.
“It was an extremely tough match,” he said after a gruelling 3 hours and 17 minutes in the Arthur Ashe stadium.
Murray has now won eight times from two sets down.