Hassan is the only athlete in Olympic history to win medals across a middle-distance event and both long-distance races in a single Games.

On the 2013 IAAF Diamond League circuit she was runner-up in the 1500 m at Athletissima with a personal best of 4:03.73 minutes and was third at the DN Galan 3000 m with a best of 8:32.53 minutes—this time ranked her the fourth-fastest runner in the world that year.

[29][30] Hassan became a Dutch citizen in November 2013, too late for competing at the 2013 World Championships, and the following month she made her first appearance for the Netherlands.

At the 2013 European Cross Country Championships she won the gold medal in the under-23 category and helped the Dutch team to third in the rankings.

[37] At the 2014 European Championships, Hassan won gold in the 1500 m and silver in the 5000 m.[8] She ended her season by winning at the 2014 IAAF Continental Cup.

She was the third female Dutch winner in the event, following in the footsteps of fellow African migrants Hilda Kibet and Lornah Kiplagat.

[46] On 17 February, Hassan set the world record for a 5 km road race stopping the clock at 14:44 in Monaco.

Olha Lyakhova was the pace setter, taking the field through the first two laps (measured at the start line, not the quarter-mile splits) in 64.26 and 63.94 (2:08.20).

As is typical for Hassan, she was last off the start line, but over the next 150 metres, slowly eased herself around the field on the outside into the marking position behind Lyakhova.

Hassan covered the last 409.344 metres in 62.20, her final time of 4:12:33 breaking Svetlana Masterkova's almost 23-year-old world record.

[51] The athletes trailing Hassan rewrote the all-time top 25 list, with Laura Weightman moving into position #15, DeBues-Stafford into #17, and after #5 all-time Tsegay faded into the pack she was followed by Rababe Arafi, Axumawit Embaye, Winnie Nanyondo and Ciara Mageean moving into positions #20–23.

[53] Hassan also won the 1500 metres race with a time of 3:51.95 (sixth place on the 1500 m all-time list), setting new championship and European records.

The second-placed finisher was Faith Kipyegonin 3:54.22, a new Kenyan national record, and the third place went to Gudaf Tsegay with 3:54.38.

Hassan lost the record two days later, however, when Ethiopia's Letesenbet Gidey achieved a time of 29:01.03 at the same stadium.

She ended a break from competing just 8 days before the World Championships in Eugene by running at the low-key Stumptown Twilight Meet in Portland where she won the 5000 metres in 15:13.41.

Despite this, Hassan caught up with the slowing lead quartet with four kilometres to go and prevailed in a sprint finish on the final straight, four seconds ahead of Alemu Megertu.

[64][65] On 3 June, just 41 days after her marathon debut, Hassan made her return to outdoor track at the FBK Games in Hengelo, Netherlands.

[67] At the 2023 World Championships, Hassan was leading the 10,000 metres until the final 100m when she stumbled and fell after getting tangled with Gudaf Tsegay, she ended up finishing 11th.

She also became the first Olympic athlete male or female to win medals in these three events at the same Games since Czechoslovak Emil Zátopek in 1952.

[78] Sifan Hassan achieved her first career breakthroughs while under the direction of Dutch national coach Honore Hoedt.

Hassan decided to search for other coaching options at the end of 2016 after injuries hampered her buildup to the Rio Olympics.

Beginning in 2017, Hassan moved to the United States to be coached by Alberto Salazar at the Nike Oregon Project.

In October 2019 Salazar began serving a four-year ban from athletics for doping violations dating from before he started coaching Hassan.

According to court rulings which upheld Salazar's ban, there was "no evidence put before the CAS as to any effect on athletes competing at the elite level within the Nike Oregon Project.

"[81] The aftermath of Salazar's ban caused the Nike Oregon Project to dissolve leading Hassan and Kejelcha to form a new training group under coach Tim Rowberry.