When the war began, Lothar von Richthofen was an officer in the cavalry. Following his brotherManfred von Richthofen's example, he transferred to the German Air Service in 1915 and was assigned to Jasta 11 on March 6, 1917. Under the watchful eye of his older brother, Lothar scored 24 victories in 47 days and was credited with shooting down English aceAlbert Ball on May 7, 1917.

On May 13, 1917, Lothar was badly wounded in a dogfight with a B.E.2e but recovered and assumed command of Jasta 11 on September 24, 1917. Wounded again on March 13, 1918, he crash landed his Fokker DR.I after being shot down by Australian ace Geoffrey Hughes. The following month, he was still in a hospital bed when he learned of his brother's death.

In the summer of 1918, Lothar returned to duty and achieved ten more victories by the end of the war. Scoring his final victory on August 12, 1918, he shot down a Sopwith Camel flown by English ace John Summers. The following day, Lothar was seriously wounded for the third time when his Fokker D.VII was shot down over the Somme by another Sopwith Camel.