Jessica won two AAA U15 titles and the English Schools junior high jump in 2000 and was second in the English Schools Inters high jump and heptathlon. She missed the British U15 record for heptathlon by just 14 points finishing in second place in a junior international in Switzerland. At U17 level she won the AAA high jump in 2001, with a best that year of 1.75 and in 2002 she won the English Schools Inters high jump with 1.80.
In 2003 Jessica won the AAA indoor pentathlon and outdoors won the Northern Junior high jump and AAA Junior 100mh. The World Youth Championships saw her finish in fifth place with 5311 points.
In 2004 she led the World Juniors Championships having achieved a pb of 13.57 for 100m hurdles, and although she finished 8th she added 178 points to the 5364 pb she had set earlier in the year. She ended 2004 with silver medals at 100m hurdles and high jump at the Commonwealth Youth Games and made further progress by winning the 2005 AAA Indoor pentathlon. In this competition she set pbs in five of the seven events, including 1.85 for high jump.
She then set UK junior records for heptathlon at both her championships events: taking gold at the European Junior Championships and bronze at the World University Games.
In 2005 she was ranked amongst the best world juniors at heptathlon and was ranked second at the100m hurdles and 11th at high jump.
2006 was another year of great achievements with her first senior medal, a bonze medal at the Commonwealth Games in Melbourne. In amassing a score of 6269pts she set four personal bests.
At the European Championships in Gothenberg, Jessica finished in eighth place, just 3 points behind Kelly Sotherton, and set personal bests in the shot putt and 200m.
Having established herself as a world-class multi-eventer, Ennis set a PB at the European Indoor Championships, finishing sixth with 4716 points, before improving her heptathlon lifetime best three times in the outdoor campaign.
In Desenzano del Garda, Ennis equalled the British high jump record of 1.95m, en-route to victory, amassing 6388 points and then defeated team-mate Kelly Sotherton at the European Cup in Szczecin, adding another 11 points to her PB. In Osaka at the World Championships, Ennis finished outside of the medals finishing fourth, albeit in a lifetime best of 6469 points.
However, in her first outdoor competition of 2009 she recorded a lifetime best of 6587 points to win by 510 points in Desenzano del Garda, Italy. This was a fantastic performance after she was forced to miss the Olympic Games in Beijing with a stress fracture in her ankle.
In August 2009, Jess won the gold medal in the heptathlon at the World Athletics Championships in Berlin. She set the third highest first day points score ever and finished with a personal best points score of 6731.
At the 2010 European Championships in Barcelona she again started as favourite and produced the goods again in a closely fought competition winning in a life-time best of 6823 points only eight points behind the UK best. Earlier in the year she had also secured the IAAF World Indoor Pentathlon title in Doha with a score of 4937 points and also produced an early season victory in the heptathlon with a points total of 6689 in Gotzis in poor conditions. At the 2011 World Championships in Daegu she had to settle for the silver medal after a below par javelin cost her valuable points.
On 27 May 2012, at the Hypo Meeting in Gotzis, Austria, Jess finally beat Denise Lewis's British heptathon record with a 6906 points total.
Jessica went into the 2012 Olympics as favourite fot the heptathlon gold, and she certainly didn't disappoint. She started as she meant to go on with a PB of 12.54 seconds on the 100m hurdles. She also set new PBs of 22.83 for the 200m and 47.49n in the javelin. Going into the final event of the heptathlon, she only needed to finish within 13 seconds of the second place athlete to win gold. Sitting in 3rd place in the race with 100m left she produced an impressive burst of speed to win the race and heptathlon gold.