She began her international junior career in
cross country running in 2001, highlighted by 33rd place at the
2003 IAAF World Cross Country Championships in her debut championships in Lausanne. She won a steeplechase silver at the
2006 European Cup and also competed in the event at the
European Athletics Championships, being eliminated in the heats. She took the under-23
silver medal at the
2006 European Cross Country Championships at the end of the year. She was fourteenth in the senior race at the
2007 IAAF World Cross Country Championships – the second best European performer behind
Jessica Augusto (12th). She made her global track debut at the
2007 World Championships in Athletics and was twelfth in the steeplechase final. In preparation for the world competition, she ran at the
Almond Blossom Cross Country in March 2011 and took second place. She managed 16th place at the
2011 IAAF World Cross Country Championships in
Punta Umbría. She ran a personal best of 15:31.26 minutes for the
5000 metres at the
FBK Games in June then ran a steeplechase best of 9:37.60 minutes at the
Adidas Grand Prix in New York. Britton represented Ireland on the track at the
2011 World Championships in Athletics, narrowly being eliminated in the first stage of the steeplechase. In November she came third at the high-level
Cross de l'Acier cross country race. In December 2011, she won the gold medal at the
2011 European Cross Country Championships. She began 2012 with wins at the
Great Edinburgh Cross Country and
Antrim Cross Country. She expressed her disappointment that the World Cross Country would not be held that year, and stated that she would focus on track running that year instead. In December of that year she became the first woman to defend the continental cross title claiming victory at the
2012 European Cross Country Championships. In January 2013, Britton retained her Great Edinburgh Cross Country and Antrim Cross Country titles. On 3 March 2013, she won a bronze medal in the
3000 metres final at the
2013 European Athletics Indoor Championships in
Gothenburg. She placed 13th at the
2013 IAAF World Cross Country Championships, being Europe's top performer in the race and one of only two non-Africans in the top 15 (alongside
Neely Spence). Her first success under her married name was a team bronze at the
2015 European Cross Country Championships, where she narrowly missed an individual medal in fourth place behind
Karoline Bjerkeli Grøvdal. She finished 5th in the
2019 Chicago Marathon. In 2019, she competed in the
senior women's race at the
2019 IAAF World Cross Country Championships held in Aarhus, Denmark. She finished in 18th place. In December 2024 she ran her personal best of 2:23:46 in
Valencia at the age of 40. She finished in 10th place at the
2025 New York City Marathon with a time of 2:27:00, winning the masters division prize of $3,000. ==International competitions==