Klosterhalfen attended Städtisches Gymnasium am Oelberg in Königswinter-Oberpleis, she had A-levels in German and Social Science.

[11] After Lewis, she was trained by Sigrid Bühler and DLV women's coach Sebastian Weiß, who gave her the nickname "Koko".

[18] In her leisure time, she likes to play the piano and the western concert flute, in addition to ballet.

[22] In December, Klosterhalfen won the European Cross Country Championships in the U20 women's division in Hyères, France.

[27] In July, Klosterhalfen won the bronze medal in the 3000 m at the World Junior Championships in Bydgoszcz, Poland in a time of 8:46.74, a German U20 record.

[29][30] In December, Klosterhalfen won for the second consecutive time (12:26 minutes) the U20 women's race at the European Cross Country Championships in Chia, Italy.

[31] On 31 December, she competed at the Bitburger-Silvesterlauf (de) in Trier, Germany and won the 5 km road race with a time of 16:05 minutes.

[33][34] Klosterhalfen's first major senior medal came in the 1500 m at the European Indoor Championships in Belgrade in March, when she came second to Laura Muir.

[39] Later that month, Klosterhalfen competed again in the Diamond Race meet at the Müller Grand Prix Birmingham in the UK.

At the European Cross Country Championships in Šamorín, Slovakia in December, Klosterhalfen placed second (20:25 minutes) on a 6.28 km course in the U23 women's division.

[41] She returned at the last day of the year to the Bitburger-Silvesterlauf 5 km for her second time, winning the event with a new personal best of 15:34 minutes.

[42] At the German Indoor Championships in Dortmund on 18 February, her 21st birthday, Klosterhalfen set a new national record in the 3000 m with 8:36.01.

[17][53] In August at the German Championships in Berlin, Klosterhalfen set a new national record in the 5000 m with a time of 14:26.76, a massive improvement on her former personal best of 14:51.38, putting her thirteenth on the world all-time list.

[54] Later that month, she broke in windy conditions the German national record for the mile run and claimed her first victory in the Diamond League.

The discipline final winner became Hassan in 3:57.08, Klosterhalfen beat the fourth-placed world record holder Genzebe Dibaba by 1.84 seconds.

Third-placed, Klosterhalfen's time of 14:29.89 beat 2017 world champion in the event Hellen Obiri, who came in fourth, by 4.01 seconds.

[65] At the NOP Klosterhalfen was trained by Salazar's former assistant Pete Julian, who maintained on German television, "We know, we do everything right".

[67] Regarding the closure of Nike's Oregon Project, Klosterhalfen stated: "It's a first and important decision, specially to protect the active athletes and what they have achieved.

[68] On 24 October, the Portland paper "The Oregonian" stated that Pete Julian likes to continue supervising seven athletes, formerly members of the Nike Oregon Project, among them Klosterhalfen.

Furthermore, the article highlighted that no NOP athlete has ever failed a drug test nor been credibly accused of doping.

The race was won by Jessica Hull (also coached by Pete Julien) in 4:04.14, an improvement on the Oceanian indoor record by more than two seconds.

[71][72] As the defending winner, Klosterhalfen placed second in the indoor mile run at the Millrose Games in February.

Klosterhalfen won the competition at the Boston University Last Chance Invitational meet in 14:30.79, which placed her fourth in the world all-time ranking.

[86] With her first 10,000 m race at the Trials of Miles Texas Qualifier in Austin, TX in February, Klosterhalfen set a new national record.

[2] The following month in home Trier, Klosterhalfen broke yet another national record, this time in the less common 2000 metres distance which had stood since 1985.

[94] It was the first double start at a major championships of her career as she also competed in the 10,000 m event three days earlier to finish fourth with a season's best.

[95][93] In October, on her debut at the distance, Klosterhalfen won the Valencia Half Marathon with the third-fastest time ever set by a European woman of 65:41, becoming the first female non-African winner of the event since 2003.

[96][97] She competed at the European Cross Country Championships in Italy on 11 December, finishing a close second behind Karoline Grøvdal on a hilly and demanding 7.662 km course near Turin, leading Germany to their first ever team gold.

[100] At the Istanbul European Indoors the following month, Klein turned the tables in the last 100 m with a gold in 8:35.87 while Klosterhalfen claimed silver in a time of 8:36.50.