In 1891, Moșoiu moved to Romania and joined the Romanian Army ranks. He commanded companies in the 9th Regiment Romanați and the 9th Battalion Vânători in Ploiești, the 1st Infantry Battalion of the 30th Regiment Muscel, the 6th RegimentMihai Viteazul in Bucharest, and the 7th Battalion Vânători in Galați. He saw action in the military campaign in Bulgaria during the Second Balkan War and on October 1, 1913, he was appointed commander of the 30th Regiment Vânători at Câmpulung, holding the rank of Lieutenant-Colonel.
On 1 December 1918, the Union of Transylvania with Romania was officiated by the elected representatives of the Romanian people of Transylvania. Later that month, Romanian units reached the line of the Mureș River, which was the demarcation line agreed upon by the representatives of the Allied powers and Hungary. On 10 December, a unified command of the Romanian army in Transylvania was established; its headquarters were at Sibiu, with Moșoiu in command. On 12 March 1919 he was appointed Military Governor of Transylvania. On 21 March Béla Kun led a successful communistcoup d'état in Hungary; he formed a social democratic, communist coalition government and proclaimed the Hungarian Soviet Republic. General Gheorghe Mărdărescu was in command of the Romanian army in Transylvania, with General Moșoiu in command of the Army Group North. By 18 April, the first elements of the Romanian offensive were completed and the Hungarian front was broken. Moșoiu's troops took control of western Transylvania, including the cities of Șimleu Silvaniei, Beiuș, and Oradea by 20 April, and then Salonta, Carei, and Satu Mare, pressing on to the Tisza River. The Hungarian forces retreated to Szolnok and from there across the Tisza, establishing two concentric defense lines. Between 29 April and 1 May, the Romanian Army broke through these lines, establishing control on the entire east bank of the Tisza. On 2 May, Hungary sued for peace, with Kun prepared to recognize all of Romania's territorial demands. General Moșoiu became the governor of the military district between the Romanian border and the Tisza River. After repelling attacks by the Hungarian Army in July 1919, the Romanian Army crossed the Tisza River at the end of the month and advanced towards Budapest. On 2 August, Kun fled Hungary towards the Austrian border and eventually reached the Soviet Union. On 3–4 August some 400 Romanian soldiers with two artillery guns took hold of the city. Soon after, the bulk of the Romanian troops arrived in the Budapest and a parade was held through the city center in front of their commander, General Moșoiu. Romanian forces continued their advance into Hungary, stopping at Győr; they occupied all of Hungary with the exception of an area around Lake Balaton. Moșoiu was named commander of the Romanian military garrison in Budapest and Military Governor of the Hungarian territory to the west of the Tisza River.
After the War
In December 1919 he retired from the Army, joined the National Liberal Party, and was named Minister of War in the Alexandru Vaida-Voevod cabinet, serving in this position until March 1920. Moșoiu died in Bucharest on 15 August, 1932. He is buried at Bellu Cemetery, in Bucharest. On the 75th anniversary of his death, a bust of Moșoiu was erected in Bran, Brașov County in 2007; the inaugural ceremony was attended by then-President Traian Băsescu.