The 26-year-old needed just over an hour to dispatch a distinctly off-colour Oceane Dodin of France 6-3, 6-0.
Watson will now face either Belgian 16th seed Elise Mertens or Varvara Lepchenko of the USA as she bids to reach the third round for the first time.
Watson arrived at Roland Garros having lost nine of her previous 10 matches, and looked nervous as she dropped her opening service game. But Watson broke back for 2-2 and as Dodin’s error count began to creep up she went on to take the opening set.
Dodin’s game then fell away completely to virtually gift Watson a free pass into the next round.
She needed five match points to get over the line but eventually wrapped up a morale-boosting win.
“I started a little nervous,” said Watson. “I was feeling really confident coming in, and loosened up. I played really, really well.
“It was tough to close it out. You could feel the rain coming up. Throwing the ball up to serve I kept getting water in my eyes.
“I was happy with my level throughout the whole match. Despite my season, I was feeling so confident coming into the match because I’ve done everything to prepare, to the best of my ability.”
Second seed Caroline Wozniacki cruised into the second round with a straight-sets win over Danielle Collins.
The Australian Open champion brushed aside her American opponent 7-6 (7/2,) 6-1 on Court Philippe Chatrier.
Having entered a grand slam for the first time since she won one, the Dane said: “It feels great. A little bit less pressure. I kind of feel like I have one grand slam now, so no matter what happens, no-one can take that away from me.
“I’m just out here trying to enjoy every match and every moment of it.”
Eighth seed Petra Kvitova looked to be in trouble when she dropped the first set against Paraguay’s Veronica Cepede Royg.
But the two-time Wimbledon champion hauled herself level and then edged a nervous final set to win a two-hour marathon 3-6, 6-1, 7-5.
“It was pretty tough,” the Czech said. “I relaxed in the second and tried to focus on every point. It was the same in the third but I managed to get the break.”
There was a high-profile casualty, though. Former world No 1 Victoria Azarenka, the 2013 semi-finalist, lost 7-5, 7-5 to 22-year-old Czech Katerina Siniakova.
Latvia’s Anastasija Sevastova, seeded 20th, also failed to make it through round one as she tumbled out 4-6, 6-1, 6-3 to Colombian qualifier Mariana Duque-Marino.
Maria Sharapova’s belated return to the tournament was delayed by the weather.
Last year the two-time champion in Paris had just returned from a 15-month doping ban, but had not built her ranking high enough to qualify and was not offered a wild card.
Now Sharapova is back in the world’s top 30 and seeded 28. However, her first-round clash with Richel Hogenkamp of Holland was held back until today with light rain falling and the light failing.