Born in Munich, Bavaria in 1888, Schleich, enrolled in the Bavarian Army cadet program shortly after school and in 1909 was commissioned into the 11th Bavarian Infantry Regiment.
Before the outbreak of the First World War, Schleich was plagued by medical problems and was released from active duty.
He volunteered again and in August 1914 was badly wounded in the Battle of Lorraine.
While Schleich was recovering from his wounds of August 1914 he decided to volunteer for the Royal Bavarian Air Service and was accepted for training as an observer.
After service with FEA 1 on two-seaters, he applied for pilot training and qualified in September 1915.
In October 1915 he joined FA 2b, and in January 1916, during an observation flight, Schleich was wounded in the arm by an exploding anti-aircraft shell. Instead of returning to base, while still in the air he had his crewman bandage his wound and then completed his assignment. He was decorated with the Iron Cross First Class for this action. The wound meant he did not see further active service until September, when he assumed command of Fliegerschule 1.
When Leutnant Erich Limpert, his best friend on the Jasta, was killed in a dogfight, Schleich ordered his plane to be painted all black. This black plane soon led to Schleich being dubbed 'The Black Knight'.
At war's end Schleich was commanding Jasta 21 and his total score was 35.
Schleich's decorations would be;
Iron Cross of 1914, 1st and 2nd class (December 1917)
Order Pour le Mérite (December 1917)
Knight's Cross of the Military Order of Max Joseph (June 1918)