1998–2001: High school years

She is an alumna of the Long Beach Wilson High School where she ran and until 2017 held the national high school record for the 300 m hurdles.^[2] She ran on the 4x400 m relay team that set the national record in 1998, ran the second fastest time in history in 1999, and then broke its own national record in 2001^[3] (since surpassed by cross town rival Long Beach Polytechnic High School in 2004).^[2] She also competed in the 100 meter hurdles, winning the CIF California State Meet in 2001, on the 4x100 m relay team, champions in 2001,^[4] as well as many of the sprint medley teams. In 1999 and again in 2001, she was named the national Girl's "High School Athlete of the Year" by Track and Field News. She is the only person to be so honored twice, non-consecutively.^[5]

2001–2005: Collegiate years

After High School, Lashinda attended the University of South Carolina to work under Curtis Frye. Her top times in college were as follows: 55 m H: 7.80; 60 m H: 8.32; 100 m H: 13.35; 400 m H: 54.70; 400 m: 51.38; 800 m: 2:13.77.^[6] While at South Carolina, Demus won the World Junior Championship in 2002, the NCAA Indoor Championship at 400 meters in 2004, the first of three National Championships^[7] and a silver medal in the 2005 World Championships. Demus also was a member of the school's first NCAA team national championship when the women's track and field team won the 2002 NCAA Outdoor National Championship.

2008–2011: World champion at 28 years old

Leaving behind the memory of failure to qualify for the 2008 Summer Olympics in Beijing, she won the 2009 US Championships in the 400 m hurdles, with a world-leading 53.78 seconds, gaining herself a place at the 2009 World Championships in Athletics.^[8] She improved upon this with a time of 52.63 seconds at the Herculis meeting in July. This was a meeting record and was then the fourth fastest time ever for the event.^[9] With that time she was the favorite to win the World Championships but faltered over the last two hurdles as she was passed by Olympic gold medalist Melaine Walker of Jamaica who was en route to the #2 time in history, leaving Demus to take home a second silver medal. However, Demus got her revenge at the 2011 World Championships in Athletics in Daegu, South Korea, when she won the gold medal in 52.47 seconds, a new American Record and the third fastest time in history. She beat reigning Olympic Champion and defending World Champion Melaine Walker, who finished second. Heavy favorite Kaliese Spencer who had set the fastest time in 2011, could only finish fourth behind Demus, Walker and 2010 European Champion and 2004 Olympic 400 m bronze medalist Natalya Antyukh.^[10]

2012 Summer Olympics

At the 2012 Summer Olympics held in London, Demus originally won the silver medal for the women's 400 m hurdles behind Russia's Natalya Antyukh.^[11]

In 2019, following a re-test of doping samples, Antyukh was disqualified with all her results 2013 onward deleted but her 2012 Olympic results were initially not affected.^[12][13][14] However, following further re-tests in October 2022, Antyukh's results from July 15, 2012, onward were retroactively voided. On 20 December, it was announced that she had been stripped of her 400 m hurdles gold and Demus was upgraded to gold medal in her place, becoming the first woman from the United States to win the Olympic 400 m hurdles title.^[15][16]