She was originally trained in equestrian sports by her mother, Else Holst, but began to be coached by Danish Olympic trainer Gunnar Andersen when she became nationally competitive.

[4] She began in dressage at an early age alongside her sister, Tove Holst (later Jorck-Jorckston), but also competed in show jumping as a teenager.

[7] In September 1944, at age 23 and pregnant with her second child, Lis contracted polio, which permanently paralyzed her below the knees, as well as affecting her arms and hands.

[9] Hartel was determined to continue her equestrian career despite medical advice otherwise, and in 1947 she finished second at the Scandinavian championships, although she had to be helped onto her horse when she rode.

[13][Note 1] The mare was also not immediately a dressage prospect, based on her conformation, per Lis Hartel's daughter, Pernille Hartel-Siesbye: "Jubilee was neither noble nor beautiful, and she had a very long back.