When the war began, Lothar von Richthofen was an officer in the cavalry. Following Manfred von Richthofen's example, he transferred to the German Air Force in 1915 and was assigned to Jasta 11 on 6 March 1917. Under the watchful eye of his older brother, Lothar scored 24 victories in 47 days and was credited with shooting down English ace Albert Ball on 7 May 1917. On 13 May 1917, Lothar was badly wounded in a dogfight with a B.E.2e but recovered and assumed command of Jasta 11 on 24 September 1917. Wounded again on 13 March 1918, he crash landed his Fokker DR.I after being shot down by Camel pilot Augustus Orlebar and Bristol F.2b crew Geoffrey Hughes and Hugh Claye. 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 12 August 1918, he shot down a Sopwith Camel flown by English ace John Summers. The following day, Lothar was seriously wounded for the third time 13 August 1918 when his Fokker D.VII was shot down over the Somme by a Sopwith Camel of the 148th Aero Squadron.
A commercial pilot in 1922, Lothar was killed in a crash on a flight from Berlin to Hamburg. A plaque honors his memory in the family plot at Südfriedhof in Wiesbaden but Lothar was actually buried near his father in the Garrison Cemetery at Schweidnitz, Germany. Schweidnitz was tranferred to Poland at the end of World War II and the graves at Swidnica, Poland have been lost.