[1] In 1900, Lill voluntarily joined the Imperial Russian Army, and studied from 1901 to 1904 in Vilnius Military School where he met Johan Laidoner, the future Commander‑in‑Chief of the Estonian Armed Forces.

In October 1914 he was captured in East-Prussia by the Imperial German Army, and was held as a prisoner of war until his release in December 1918.

For his service during the War of Independence, Paul Lill was awarded Estonia's Cross of Liberty and Latvia's Order of Lāčplēsis, as well as a farm and 300,000 marks.

[3] After the war Lill continued military service, becoming initially acting, and from October 1920 permanent Chief of Staff of the Defence Forces.

[5] On 12 October 1939 Paul Lill resigned as Minister of War, citing unacceptable conditions of the Bases Treaty with Soviet Union.

[1][5] In October 1940 Soviet occupation authorities revoked Lill's retirement pension, and in December evicted him from his apartment at so-called "generals house" at Gonsiori street.

Paul Lill died in a prison camp in Sverdlovsk Oblast on 13 May 1942, his place of burial is unknown.