The American, who trains in Baltimore with Michael Phelps, stormed to the front on the second of four laps and was unchallenged as she won the gold in a time of one minute 53.61 seconds.
Frenchwoman Camille Muffat, who just held off Schmitt to win the 400 final on Sunday, took silver in a time of 1:55.58.
Australia's Bronte Barratt, a relay gold medallist in Beijing four years ago and the fastest qualifier for the final, finished third in 1:55.81, just edging out Missy Franklin for the bronze.
The 17-year-old from Colorado won the 100 backstroke gold the previous night and also picked up a relay bronze on Saturday but missed out on another medal by just 0.01 seconds.
Loughborough-based McClatchey swam 1:57.60 and was delighted with the high level of support given to her by the home crowd.
She admitted that her thoughts will now turn to the 4x200m relay, but couldn’t confirm if Rebecca Adlington will be involved.
She said: “The crowd was so amazing tonight and I want to thank them all for their support. It’s been fantastic – I’ve had such a great time.
“We’ve got a really strong relay coming up tomorrow – I’m really excited for it. I’ll rest now and re-charge my batteries.
“We don’t know if Becky is going to be swimming.”
Jemma Lowe, with a time of 2:07.37 mins, qualified for the 200m butterfly final in eighth position and admitted she was happy to be out in the far lane.
Lowe, who placed fifth in her heat, will start in lane eight and is hoping that destiny will play a part in helping achieve a medal in her second Games.
“That’s great for me as my coach didn’t want me to be in the middle lanes, so I feel quite lucky, and it’s meant to be. I’m ready for tomorrow now. The outside lane is perfect for me," she said.
“It’s really hard to get into an outside lane when it’s really fast. You don’t know in a semi-final, do you give everything you’ve got or do you save something for the final? It was perfect.
“I am capable of a 2:05.00 mins and finals aren’t normally really fast as the semi-finals. Just going to give it everything and see how it goes.
"Tomorrow I am going to give everything I have got and see what it brings."
World champion James Magnussen, meanwhile, resurfaced from his disappointing showing in the relay to top the qualifiers for the final of the men's 100 metres freestyle.
Magnussen had boasted before the Games that he would win gold medals in the blue-riband sprint and the 4x100 freestyle relay but he had a shocking lead-off leg in the relay and Australia finished only fourth.
He was in much better form on Tuesday, advancing to the final as the fastest qualifier after winning his semi in 47.63 seconds.
"It is a relief more than anything to feel what it is like to go fast. I felt really good tonight and that I still have plenty left in the tank," Magnussen said.
Nathan Adrian, who won a silver medal for the United States in Sunday's relay after beating Magnussen in the lead-off leg, was second fastest overall, posting a time of 47.97, while Brazilian world record holder Cesar Cielo was fifth overall in 48.17.
Magnussen was the overwhelming favourite to win gold before the Games after winning the world title last year but said his self-belief had been shaken by the relay result.
"I had doubts, something that I haven't done throughout my whole career," he said.
"I have always been so confident. It was probably the reality check that I needed. If I can win this one it will be for the other boys in the relay team."
And American Kathleen Hersey outpaced Chinese world champion Jiao Liuyang to top the qualifiers for the women's 200 metres butterfly final.
The 22-year-old plowed her way through four laps of the Aquatics Centre in a time of two minutes, 05.90 seconds to win her semi-final and advance to Wednesday's final as the top seed.
"I finally learned how to be relaxed and have fun with the race and not get too caught up," Hersey said.
"If you don't let yourself at least feel some of the emotion of the Olympics you are missing out on so much of it. I think I finally mastered how to get the crowd's energy behind me."
Jiao won the other semi in 2:06.10 to finish second overall while Japan's Natsumi Hoshi was third fastest and China's Liu Zige, the world record holder, was sixth.