Cavani wasted two presentable opportunities to double PSG's lead soon afterwards but he did not have to wait long to score his second goal in as many league outings this season as he ran onto Neymar's sumptuous pass and slotted beyond Johnsson.
Neymar scored a goal and provided an assist on his debut for Paris Saint-Germain, as they beat Guingamp 3-0 on Sunday.
The Brazil forward was making his much-anticipated first appearance for PSG since joining them from Barcelona for a world-record £200m transfer fee.
Jordan Ikoko was the major culprit for the hosts when he netted an own goal in the 52nd minutes to the disappointment of his coach.
Neymar will line up in a front three alongside Edinson Cavani and Angel Di Maria, with Javier Pastore dropped to the bench after starting against Amiens.
With Unai Emery's side already leading 1-0 thanks to Jordan Ikoko's freakish own goal, Neymar assisted Edinson Cavani with a wonderful pass before the roles were reversed eight minutes from time for the Brazilian to tap home.
Neymar had earlier provided a delightful 62nd-minute pass from which Cavani struck to make it 2-0.
3 - Games needed to see @neymarjr's first goal in...
After De Preville was beaten by Jonas Martin's 74th-minute shot, defender and captain Ibrahim Amadou then took over between the posts but was beaten by Dimitri Lienard (82, pen) and Jeremy Grimm (88).