Sanders career peaked in 2007 when she won the 400 metres at the 2007 European Indoor Championships (breaking the British Indoor record), and ran a personal best to win the silver medal at the 2007 World Championships behind compatriot Christine Ohuruogu. In the 4×400 metres relay, she won a bronze medal at the 2008 Beijing Olympics (retroactively awarded in 2017) and a gold medal at the 2012 World Indoor Championships. She also won World Championship relay bronzes in 2005 and 2007. After two years of struggling with injury, she announced her retirement from athletics in October 2014.
Sanders won a bronze medal in the 1999 European Athletics Junior Championships in Riga at 400 m hurdles. That same year she was 4th in the 400 m hurdles at the World Youth Championships. In 2000, she was 5th in the 2000 World Junior Championships in Athletics, but won gold in the Commonwealth Youth Championships. She grew up in Amersham and went to Amersham School.
In 2005, she finished 6th (4th 4 × 400 m relay) at the World Student Games.
In the 2005 Helsinki World Championships Sanders reached the semi finals. Together with Lee McConnell, Donna Fraser and Christine Ohuruogu she won a bronze medal in the 4 × 400 m relay.
She was 4th in the Commonwealth Games of 2006 in the 400 m hurdles and was part of the team that won the 4 × 400 m relay, but they were subsequently disqualified.
Since then she has focused on 400 m (due to injury issues) and rarely participates in hurdles races.
She finished 6th in the 400 m final at the 2006 European Athletics Championships in Gothenburg.
In 2007, she took gold in the 400 m competition in the 2007 European Athletics Indoor Championships in Birmingham, with a personal best and national record of 50.02 seconds. This was the 5th fastest indoor 400 m time in history. She also won a bronze in the 4 × 400 m relay.
In 2007, she had suffered knee and Achilles tendon problems, but on 27 August 2007 Nicola broke 50 seconds for the first time in her career, recording 49.77 seconds in the semi final of the 400 m at the World Athletics Championships in Osaka, Japan to move to #3 on the British all-time list. She then went on to finish 2nd in the final, behind fellow Briton, Christine Ohuruogu. Sanders set another personal best of 49.65, which puts her as the fourth fastest British athlete over 400 m, after Kathy Cook, Katharine Merry and Ohuruogu, who also set a personal best in the World Championship final.
During the final day of the championships on 2 September, Sanders anchored the British 4 × 400 m team to a bronze medal. In so doing she became the first female UK runner to break 49 seconds for a 400 metres relay leg, with a time of 48.76 seconds, beating Sally Gunnell's previous record of 49.46 seconds.
Sanders went out of the 400 m at the 2008 Summer Olympics in the third semi-final in a time of 50.71 seconds. With a tough lane draw, lane 9, and lacking the fitness that comes from an injury free run in to the season, Michael Johnson added, 'she's got a little bit more 800 metres look to me this year than she did last year, there's just not the power, the arm drive that we saw in 2007.' She was part of the 4 × 400 m relay team which finished fifth in the final although the team was later upgraded to bronze medal position following disqualification for doping offences of the teams finishing in second and fourth place.
In December 2011, Sanders was one of 12 British female sporting celebrities who posed for Clara Maidment a charity calendar in aid of Wellbeing of Women, in the lingerie of Nichole de Carle, wearing jewellery by Salima Hughes and Coster Diamonds.400 meters - 49.65 (2007 Osaka, Japan) Outdoors, 50.02 (2007 Birmingham, England) Indoors
400 meters hurdles - 55.32 (2006)