The result leaves PSG 14 points ahead of second-placed Monaco, who have five games remaining including Wednesday's home meeting with Lille.
If Monaco fail to win that, PSG will be able to celebrate a record-extending 12th French league championship triumph, and a 10th in the last 12 campaigns.
Luis Enrique's side have a potential treble in their sights, with an upcoming Champions League semi-final against Borussia Dortmund and the French Cup final against Lyon to look forward to next month.
They made short work of relegation-threatened Lorient in Brittany after Dembele gave them the lead on 19 minutes, collecting a Senny Mayulu pass and coming in from the right flank before sending a shot low into the bottom-left corner.
Nuno Mendes then cut the ball back for Mbappe to double their lead midway through the first half.
PSG were looking to extend their unbeaten run in Ligue 1 to 25 games going back to September and their only loss of the campaign so far, at home to Nice.
Lorient, in contrast, were aiming to avoid a fourth consecutive defeat and climb out of the automatic relegation places.
The visitors were without several rested regulars, including captain Marquinhos, Achraf Hakimi and Vitinha, but Mbappe and Dembele were both very much in the mood.
Mbappe produced a brilliant piece of skill to get away from Nathaniel Adjei on the touchline to the left of the box before squaring low for Dembele to tap in the third goal on the hour mark.
That took Dembele's tally of goals for the season to five, after he also netted in both legs of the Champions League quarter-final win against Barcelona.
Lorient pulled one back on 73 minutes when Benjamin Mendy crossed for Ivorian striker Mohamed Bamba to head in.
However, Mbappe completed the scoring for PSG in the 90th minute with a trademark strike across the goalkeeper from the left-hand side of the box.
That took Mbappe to 43 goals for the campaign in all competitions in 43 appearances, with 26 of his goals coming in Ligue 1.
