While Cox, 25, took the gold, Britain’s newly crowned most decorated female paralympian of all time Dame Sarah Storey finished in fourth place in the same C4-5 event.
Speaking to broadcaster Channel 4 after the race, Cox said: “I’m over the moon.”
She added: "I can't believe it. I'm over the moon. I have only been doing it a year and a half and being on top of the world is amazing."
GB can thrive with 'target on their backs' at Paralympics, says Love
It was another golden day at the Rio Paralympics for Team GB with Hannah Cockroft retaining her T34 100m title and Andy Lewis claiming gold in the PT2 Para-triathlon.
A handful of silver and bronze medals were also won by the Brits, with Kare Adenegan taking silver in the T34 100m, Sabine Fortune a bronze in the women’s F20 shotput, and Alice Tai a bronze in the S10 100m backstroke.
Great Britain now has 35 medals – 15 of which are gold, putting them in second place on the table.
Aside from Tai’s bronze, there was disappointment at the aquatics centre, however. 15-year-old Ellie Robinson who finished fourth, and Ellie Simmonds – Robinson’s Paralympic inspiration – who finished sixth in the S6 50m freestyle.
Christiansen: Back in the saddle and in the hunt for more Paralympic success
Becoming an athlete was never my dream, says Paralympic medallist Cockroft