His father was Roman Muśnicki, the owner of Garbów, descended from the Lithuanian Dowborów (Daubor) family (Przyjaciel coat of arms), who settled in Sandomierz during the 17th century.

He was temporarily put in charge of the staff of the Russian 1st Army on 17 January 1917, 5 weeks before the February Revolution that overthrew Tsar Nicholas II.

In the immediate aftermath of the February Revolution, Dowbor-Muśnicki continued his military career and was appointed commander of the XXXVIIIth Corps on 28 April 1917 and made Lieutenant General on 5 May 1917.

Roughly 700,000 of them were serving in the Russian military by 1917 and they began forming a Polish army to fight for a "united and free Poland" with the assent of the Provisional Government.

The reorganization process was complicated by the October Revolution of 1917, which brought Bolsheviks to power, but Dowbor-Muśnicki was able to take advantage of the new government's weakness and general anarchy to form 3 divisions in Belarus by January 1918.

In May 1918, Dowbor-Muśnicki was forced to sign an agreement with Germany that led to the disarmament and effective dissolution of the Corps by July 1918, at which point he moved to Poland.

After the armistice that ended World War I in November 1918, Dowbor-Muśnicki helped organize a new Polish army around the disbanded 1st Corps and its officers.

On 6 January 1919 he was nominated by the Supreme People's Council, the temporary ruling body of the province of Greater Poland, as the new commanding officer of all the Polish forces in the area.

Two days later he arrived to Poznań and on January 16 he officially assumed his post, replacing Major Stanisław Taczak during the Greater Poland Uprising against Germany in the disputed region.

During his service as the commander in chief of the Uprising, Dowbor-Muśnicki was responsible for almost complete reorganization of what was started as a para-military partisan force.

After the Battle of Ławica in which the Poles managed to capture the airfield, the Greater Polish Army was the fourth force in the world in number of aeroplanes available.