She started her international career with performances at the Tokyo Women's Marathon and Nagano Marathon in 2006. She ran in the 2007 Rotterdam Marathon and clocked a time of 2:33:54 for fourth place. Her first major event was 2007 All-Africa Games, where she took the half marathon silver medal behind Souad Aït Salem. A few months later she attended the 2007 IAAF World Road Running Championships and finished in eleventh, helping the Ethiopian women to the team silver medal. Following this, she went on to win at the Istanbul Marathon, recording a new personal best of 2:29:05. She did not make significant progression in 2008, with fifth-place finishes at both the Rome City Marathon and Toronto Marathon being the highlights of her year. In 2009, she started with a ninth-place finish at the Dubai Marathon with a time of 2:29:13. Victory in a personal best time of 2:24:42 at the Paris Marathon marked a new high for Baysa, having won her first IAAF Gold Label Road Race. Baysa was selected for the women's marathon at the 2009 World Championships in Athletics, but she did not match her previous form and finished in 27th place. She went on to take seventh-place finish in 2:32:05 at the Frankfurt Marathon in October. Baysa made a strong start to 2010 by becoming the first foreign athlete to win the women's race at the Xiamen International Marathon. She led the race uncontested and finished in 2:28:53. Despite cold conditions, she also won the Paris Half Marathon in March, recording a time of 1:11:05. She set her eyes on defending her Paris Marathon title in April and her front running tactics succeeded in record time: finishing in first place in 2:22:04, she beat Marleen Renders' course record which had stood since 2002 and knocked over two minutes off her previous best. She ran at the 2010 Chicago Marathon in October and started well by setting a fast pace from the beginning, establishing a significant lead by the halfway point, and though it cost her at the end, she won with a time of 2:23:40, beating her closest challenger, Desiree Davila, by two minutes, forty seconds. She entered the Delhi Half Marathon in November but managed only fifth place over the half distance. She opened her 2011 at the Dubai Marathon, but finished in fifth place some three minutes behind the winner Aselefech Mergia. Another fifth came at the 2011 London Marathon, where she was one place behind her compatriot Bezunesh Bekele. She ran in two French races in October: she was the winner of the half marathon race at the Reims à Toutes Jambes and came second to Lydia Cheromei at the Marseille-Cassis Internationale. She ran a time of 2:23:13 hours at the 2012 Dubai Marathon, which was only worth eighth place in a fast women's race. She won the Tarsus Half Marathon in a course record of 69:39 minutes in March before going on to place ninth at the 2012 London Marathon. While running at the 2012 Chicago Marathon, she drew on her experience of 2010 and did not falter in the second half of the race, racing away in the final section to beat Rita Jeptoo for what at the time was her first Chicago women's title. She ended the year with a dominant performance at the Montferland Run, winning unchallenged after breaking away from the pack in the first 5 km of the race. She won the 2016 Boston Marathon, but gave Bobbi Gibb her trophy; Gibb said in 2017 she would go to Baysa’s native Ethiopia and return it to her.
