
Moffi converted the 31st-minute spotkick after he was hauled down by Ismail Jakobs shortly after Sofiane Diop skied his shot over the bar from a good position at the other end.
Monaco had 72% of the possession in the first half but Lorient came close to doubling their lead when leftback Vincent Le Goff shaved the top of the bar with a cross-turned shot in the 43rd minute.
Defender Strahinja Pavlovic glanced a header inches wide for the visitors shortly after the break but that was as close as they came to an equaliser as they created little upfront against a packed Lorient defence.
In Saturday’s standout fixture, Paris St Germain are at home to Strasbourg with new signing Lionel Messi to be presented to the fans before the kickoff.