Whelan, the daughter of Mike and Karen Whelan, was born on 1 July 1992 in Singapore, where she lived until she was two years old. Her family then moved to Stockport, England. It was there that Whelan began gymnastics. She said: "At the age of six, after constantly cartwheeling around the house and Tesco, I persuaded my mum to take me to a gymnastic class. There, I was immediately spotted as a talent and began training at South Manchester Gymnastics Club with coach Helen Hall." A year later, when Whelan was seven, her family moved to the United States, where she trained at Flips Gymnastics Academy in Texas. In 2001, at the age of nine, she moved back to Stockport with her family and resumed training with Hall, this time at Vernon Park Gymnastics Club.
In 2005, Whelan switched gyms and began training under Sarah Attwell at the City of Liverpool Gymnastics Club, alongside Beth Tweddle. "Beth always helped me in the gym," she said. "Just the little comments help to pick you up if things are going wrong. She has the experience and she has had bad days in the gym, but it's good to know that she's gotten where she has by living through the same things that you are." After 2008, Whelan began training under Amanda Reddin.
In August, Whelan competed at the 2008 Summer Olympics in Beijing. She contributed scores of 14.325 on balance beam and 14.125 on floor toward the British team's ninth-place finish.
At the 2009 European Championships in Milan in March, Whelan placed seventeenth in the all-around final with a score of 54.325. In May, she scored 56.400 at the British Teams competition in Guildford, helping her club, City of Liverpool, win first place.
In October, she competed at the 2010 World Championships in Rotterdam, where she contributed scores of 13.900 on uneven bars, 14.133 on balance beam, and 14.066 on floor toward the British team's seventh-place finish. She placed sixteenth in the all-around final with a score of 55.499.
In the team final at the 2011 World Championships in Tokyo, Whelan scored 55.665 and Britain finished fifth, qualifying a full team to the 2012 Summer Olympics. In the all-around final, Whelan placed ninth with a score of 56.124.
At the 2012 European Championships in Brussels in May, she contributed an all-around score of 55.498 toward the British team's fourth-place finish. In event finals, she placed third on balance beam, scoring 14.333, and third on floor, scoring 14.533.
In July, Whelan competed at the 2014 Commonwealth Games in Glasgow. She contributed an all-around score of 53.366 toward England's team gold and qualified fifth into the all-around final, where she won bronze, scoring 54.699. Her teammates Claudia Fragapane and Ruby Harrold won gold and silver, respectively, for an all-English podium.