Until 2012, Diggins mainly took part in competitions of the US Super Tour and the Nor-Am Cup. She ran her first World Cup race in February 2011 in Drammen , which she finished with 46th place in the sprint. At the US cross-country skiing championships in 2011, she won gold in the sprint and bronze in the 30 km mass start race. Her best placings at the Nordic World Ski Championships 2011 in Oslo were 28th place in the sprint and ninth place in the relay. In the 2011/12 season she reached the top ten places in the World Cup for the first time with fifth place in 10 km mass start races and sixth place in the sprint. At the US Super Tour 2012 she was first in the overall standings. At the US cross-country skiing championships in 2012, she won three gold and two silver medals. The 2012/13 Tour de Ski finished in 21st place. At the Nordic World Ski Championships 2013 in Val di Fiemme , she reached fourth place with the relay and became world champion in the team sprint together with Kikkan Randall . In the following 2013/14 season , she finished the Tour de Ski in 13th place. At the U23 World Championships 2014 in Val di Fiemme , she won silver in the sprint. Her best placings at the 2014 Winter Olympics in Sochi were eighth place in skiathlon and ninth place with the relay. She finished the season in 20th place in the overall World Cup ranking. At the Nordic World Ski Championships 2015 in Falun , she won silver in the 10 km freestyle. After 38th place at the Nordic Opening in Ruka at the beginning of the 2015/16 season, she was third in the relay in Lillehammer . At the Tour de Ski 2016 she came tenth in the overall ranking. She won her first individual victory in the World Cup in Toblach in the 5 km freestyle. At the following World Cup in Nové Město she won third place over 10 km freestyle and second place with the relay. In February 2016, she came back on the podium with second place in the sprint in Lahti . At the end of the season she finished fifth in the Ski Tour Canada . She finished third in the Gatineau sprint and ran the third fastest time on the final stage in Canmore . At the end of March 2016, she became US champion over 30 km in Craftsbury . She finished the season in ninth place in the Distance World Cup and in eighth place in the overall World Cup and in the Sprint World Cup. In September 2016 she won the Merino pattern and the 10 km mass start race at the Australia / New Zealand Cup in Snow Farm . At the beginning of the 2016/17 World Cup season, she finished eighth in the World Cup mini tour in Lillehammer. She won the 5 km freestyle stage. At the Tour de Ski 2016/17 she was with six top ten placements, including second place in the skiathlon in Oberstdorf and first place over 5 km freestyle in Toblach, fifth in the overall ranking. At the season highlight of the Nordic World Ski Championships 2017 in Lahti, she won the bronze medal together with Sadie Bjornsen in the team sprint and the silver medal in the sprint. She was also fifth in the 30 km mass start race and fourth in the relay. At the end of the season she came in 16th place in the World Cup finals in Québec and reached tenth place in the sprint world cup, seventh place in the distance world cup and sixth place in the overall world cup.
In September 2017, Diggins won the Merino pattern as in the previous year and at the Australia / New Zealand Cup in Snow Farm in the mass start over 10 km classic. After finishing 12th in the Ruka Triple at the beginning of the 2017/18 World Cup season, she came third overall in the 2017/18 Tour de Ski . She achieved five top ten placements, including third place in the final stage in Fiemme Valley and in the pursuit race in Lenzerheide . At the end of January 2018, she won her fourth World Cup victory in the 10 km mass start race in Seefeld in Tyrol . At the season highlight, the Winter Olympics in Pyeongchang , she came in the top ten in all competitions. In the 30 km mass start race, she was seventh, in the sprint sixth, over 10 km freestyle, in the skiathlon and fifth with the relay. Together with Kikkan Randall , she won the gold medal in the team sprint. In March 2018, she came third in the sprint in Drammen and second in the 30 km mass start race in Oslo . At the World Cup final in Falun she came second. She ran the fastest time in the final stage and finished the season in sixth place in the Sprint World Cup, third in the Distance World Cup and second in the overall World Cup. In summer 2018 she won two races at the Australia / New Zealand Cup in Snow Farm and for the third time in a row the Merino pattern. At the beginning of the 2018/19 World Cup season, she finished 12th in the Lillehammer Triple and took three third places in the 2018/19 Tour de Ski , sixth place in the tour standings. In February 2019, she took her sixth World Cup victory in the sprint in Cogne . Her best placings at the following Nordic World Ski Championships in Seefeld in Tyrol were fifth place in the team sprint and with the relay and fourth place in the 30 km mass start race. In March 2019 she came in third place over 10 km freestyle in Falun and 14th place at the World Cup finals in Québec and finally reached seventh place in the sprint world cup and sixth place in the overall and distance world cup. At the end of August 2019 she won the Merino pattern for the fourth time.
After finishing fifth in the Ruka Triple at the beginning of the 2019/20 season, Diggins was second in both skiathlon and relay in Lillehammer and third in Davos over 10 km freestyle. She finished the Tour de Ski 2019/20 with four top ten finishes, including third place in the sprint in Val di Fiemme, ninth place. This was followed by third place in the sprint in Oberstdorf and sixth place in the ski tour. She finished the season in 11th place in the Sprint World Cup, eighth in the Distance World Cup and sixth in the overall World Cup.