Alfred had claimed a first-ever medal for her tiny Caribbean island nation when she won the 100m on the weekend, but she had no response when Thomas hit the bend and powered away down the final straight at the Stade de France.
The US took gold and bronze either side of Alfred, as Thomas clocked 21.83sec for the victory and Brittany Brown rounded out the podium.
"I don’t think it could have gone any better," said Thomas, who added that she had taken inspiration from watching teammates Noah Lyles and Sha'Carri Richardson, and even Alfred in the men's and women's 100m.
"My coach told me the only thing I need is to get the lead and finish strong and I did that."
There were no complaints from Alfred.
"I did feel ready for the 200m tonight," she said. "I feel good, no complaints at all.
"First Olympics, to go back with gold and silver, I can't ask for more than that."
It is the first time since the Montreal Games in 1976 that there were no Jamaican sprinters on the podium of either the women's 100 or 200m.
There was a huge upset earlier in the evening when Jakob Ingebrigtsen and Josh Kerr were deprived of victory in the men's 1500m by fast-finishing American Cole Hocker.
The race had been billed as a crunch set-to between the Norwegian gold medallist from the Tokyo Olympics and Britain's reigning world champion.
But in the end, Hocker raced through on the inside for a totally unexpected victory in an Olympic record of 3min 27.65sec.
Kerr bagged silver in a British record of 3:27.79, while Ingebrigtsen came fourth after being passed by another American, Yared Nuguse, who clocked 3:27.80
Hocker might be a relative unknown on the circuit, but the American insisted becoming Olympic champion had been his season's goal.
"I wrote that down and I repeated it to myself even if I didn't believe it," the 23-year-old said.
"If they let me fly under the radar, then so be it. I think that might've just been the best."
'100 metres too far'
Ingebrigtsen complained that it was "just 100 metres too much", having led the field out at a fast pace over the opening 800 metres.
"Of course it's a tactical error," he said.
World Athletics president Sebastian Coe, who himself won two Olympic 1500m golds for Britain, had predicted the final could be a "race for the ages" given the bitter rivalry between Ingebrigtsen and Kerr.
And so it proved, but not quite as Coe had foreseen as the unfancied Hocker stole the show with his late surge.
There was more hammer throw joy for Canada as world champion Camryn Rogers won the women's competition with 76.97 metres.
Her victory came just 48 hours after teammate and fellow world champ Ethan Katzberg secured the men's hammer gold. The pair embraced trackside, Rogers draped in a Canadian flag.
There was no surprise in the men's long jump as multiple global medallist Miltiadis Tentoglou retained the men's Olympic long jump title.
Tentoglou managed a best of 8.48 metres for his second gold, having also won at the Tokyo Games three years ago.
Jamaica's Wayne Pinnock took silver with 8.36m while Italy's Mattia Furlani claimed bronze, just 2cm adrift.
There was a second Olympic record on the night as Bahrain's world champion Winfred Yavi won the women's 3000m steeplechase in 8min 52.76sec.
Uganda's defending champion Peruth Chemutai took silver in a national record of 8:53.34 and Kenya's Faith Cherotich claimed bronze in 8:55.15.
American reigning champion Sydney McLaughlin-Levrone and Femke Bol of the Netherlands set up a mouth-watering final in the women's 400m hurdles after each easily won their semi-finals.
It also means Bol remains on course in her audacious bid for treble gold, having already anchored the Dutch quartet to 4x400m mixed relay glory. Her third event will be the women's 4x400m relay.
Netherlands and Australia light up velodrome
The Netherlands smashed their own world record twice in defending their Olympic men's team sprint gold, while Australia fired a warning shot by obliterating the team pursuit mark.
Dutch stars Harrie Lavreysen, Roy van den Berg and Jeffrey Hoogland flew round the Saint-Quentin-en-Yvelines velodrome in a blistering 40.949secs to consign Britain to silver. Australia took the bronze.
That time followed the trio thrilling the capacity crowd with a new world best 41.191 in the heats to better the 41.225 they set at Berlin in 2020.
"It's crazy," said Lavreysen, the world's top sprinter.
"Of course we were going for the gold medal but breaking world records and going under 41, that was really special.
"The track is really fast and we thought we needed to break the record to become Olympic champions."
Lavreysen is also heavy favourite in the individual sprint and a medal hope in the keirin.
Asked if he felt unbeatable, he replied: "It feels nice going into this week, so let's see."
The same three won Olympic gold in Tokyo three years ago, ending the dominant reign of the Jason Kenny-led British team that owned the title in Beijing, London and Rio.
Once again Britain - Ed Lowe, Hamish Turnbull and Jack Carlin - were no match for the speed, strategy and power of a tight Dutch team that has been dominant since 2018 when they claimed their first world title.
Since then they have won four of the subsequent five world championships and now back-to-back Olympic gold medals.
"We came into this expecting to fight for third so as soon as we got into the gold final, all stress was off," said Turnbull.
"We just tried to express what we could do."
In the men's team pursuit, Australia beat Italy in a new world record 3:40.730 to dismantle the previous mark of 3:42.032 and make the gold medal race.
That was set by Italy on their way to the title in Tokyo in the most prestigious event of the track cycling programme.
Extra boost
The Australian team of Oliver Bleddyn, Sam Welsford, Conor Leahy and Kelland O'Brien will meet Britain for gold.
"I think it will be fast again tomorrow," said Welsford. "With the conditions and with the level of an Olympic final I think you have to find that extra boost."
Britain - who own the most Olympic medals in men's team pursuit with 15, winning at Beijing, London and Rio before slumping to seventh in Tokyo - beat Denmark to set up the clash.
There have so far been eight world records on the opening two days of track competition, with five new marks set in the women's sprint on Monday, led by Britain who won gold.
New Zealand were quickest through women's team pursuit qualifying in 4:04.679, less than half a second outside Germany's 2021 world record.
They will meet fourth-placed Italy next while Britain face a United States team boasting Paris road race gold medallist Kristen Faulkner.
The winners of those heats progress to the gold medal showdown.
In an upset, defending champions Germany only came fifth in 4:08.313, leaving their title defence in tatters.
Britain won gold when the event was added to the Olympics in London in 2012 and defended their title in Rio, but they were relegated to second by Germany in Tokyo.
