The Colombia international striker suffered a serious knee injury in January that saw him miss the remainder of the Ligue 1 season as well as his country's World Cup campaign in Brazil.
After featuring for Monaco in pre-season - scoring the only goal in a 1-0 success over Arsenal in the Emirates Cup - Falcao played in a competitive match for the first time in seven months when Leonardo Jardim introduced him from the bench as a 59th-minute substitute for Lucas Ocampos at Stade Louis II.
The 28-year-old clinically converted a spot-kick 12 minutes from time to level the scores at 1-1, only to see the visitors snatch all three points with a breakaway goal from Valentin Lavigne in the 87th minute.
Lorient had gone ahead in the ninth minute after Aymen Abdennour's foul on Vincent Aboubakar allowed the striker to coolly tuck home from 12 yards.
Monaco then lost Ricardo Carvalho to a second yellow card in the 68th minute before Falcao's goalscoring return was overshadowed by Lavigne's sucker punch.
In the other match to be played on Sunday, Hubert Fournier's reign at Lyon got off to a winning start courtesy of a 2-0 home success over Rennes.
Second-half goals from Steed Malbranque and Alexandre Lacazette - previously linked with a move away from the club - ensured Fournier's tenure began strongly.
