Originally a heptathlete, the 23-year-old Yorkshire-born athlete moved over to two wheels in 2013 - stepping on to the international competitive scene just 18 months ago.
Her sister Amy, 24, said she has “been glued to the TV” while her sibling has been racing in Rio - saying the pair are very close and like “best friends” as there is just 15 months between them.
“I am just in shock - I think if anyone ever said to her you are going to get a bronze medal she would not have believed it,” she said.
“When she got on the track she looked like she owned it and had been on there for years.”
She said her sister has in the past struggled with the tactics of the sport, but said she rode the race “like she owned the track”.
From Barwick-in-Elmet, near Leeds, Amy said her sister’s win seems “unreal” and will not believe it until she has Facetimed Katy.
“I was speaking to her before and once it became she was riding for the bronze medal - she knew the worst she could do was fourth,” Amy added.
“She wanted that medal so bad, as she did that final push to the line she was reaching for everything she had in her - she will be so happy.”
Amy continued: “I think she has really put her name on the map with this performance, I think she is definitely going to be a star of the future.”
Her former athletics coach Toni Minichiello, who also worked with Jessica Ennis-Hill, spotted Katy’s potential for cycling during Wattbike testing.
Talking to the BBC Marchant said: “I cannot believe it - literally. I am just so, so happy. All the hard work and the tears and the sweat has paid off and I could not be happier.
“I think I owe Toni a massive thank you for everything,” she added.
“To finish with a bronze medal, I genuinely cannot believe it.”