Thomas took bronze in Tokyo and silver in last year’s world championships, but she was in control throughout the final, coming home in 21.83 seconds.
Alfred, who claimed Saint Lucia’s first Olympic medal when she won the 100m on Saturday, was a clear second in 22.08, with Brittany Brown taking bronze for the U.S. in 20.20.
There was more frustration for British duo Dina Asher-Smith and Daryll Neita who finished fourth and fifth.