At the 2007 edition held in Ostrava, she was second with 4.25 m. In 2008, the 18-year-old won the bronze medal with 4.25 m at the World Junior Championships in Bydgoszcz, Poland.

Brewer at Arizona State University while training under the guidance of 2000 Olympic pole vault champion Nick Hysong.

In 2010, under the guidance of Toby Stevenson, she tied for fifth place (4.30 m) at the NCAA Division I Indoor Championships, was the Pac-10 Conference champion and tied for fourth (4.25 m) at the NCAA Division I Outdoor Championships, after breaking the school record multiple times.

In the 2012 outdoor season, she was again the Pac-12 Conference champion in Eugene, Oregon, where she broke her own school record with 4.48 m. A month later she became the NCAA champion in Des Moines, Iowa, clearing 4.45 m. Her season's best, 4.51 m, achieved in July in Livermore, California, is the Greek under-23 record.

She competed for the first time at the European Athletics Team Championships 1st league, held in Tallinn, representing Greece and winning with a 4.55 m jump.

She managed to improve her PB again at the Glasgow Diamond League in with a jump at 4.65 m. One week before the European Championships she set another personal best at 4.71 m, equaling the Greek national record.

[10] In the 2016 indoor season, she set a national record with a huge leap of 4.90 m at the Millrose Games, which ranked her at fourth place of all time in the event, tied with Demi Payne who cleared the same height at the same meeting.

She won the Diamond League title for a third consecutive year and completed the season with a second place (4.85 m) behind Anzhelika Sidorova, representing Europe at the Continental Cup.

In 2020, as the COVID-19 pandemic shut down most public sports events, Stefanidi, along with Katie Nageotte and Alysha Newman, took part in an online event conceived by Renaud Lavillenie and organized by World Athletics called "The Ultimate Garden Clash".

At their local tracks, athletes had to collect as many vault clearances in two 15-minute periods as possible with the bar set at four metres.