On September 28, 1944, James and his unit left the United States for Italy, arriving eight days later on October 6. For the remainder of US Army operations in Italy, Warrant Officer Johnson would see heavy combat as part of the 599th FA Battalion, fighting in the Rome-Arno, Po Valley, and North Apennines campaigns. The 105mm howitzers, which made up the 599th, were essential in supporting infantry units that engaged the enemy on the front lines.

For more than seven months, James and the 599th drove across Italy, with their artillery playing a significant role in supporting infantry elements. Some of the heaviest combat James would endure during the war took place in April of 1945, during the unit's decisive final offensive in Italy as the war drew to a close. Johnson and the 599th assisted in providing much-needed fire to the 442nd Infantry during the ferocious fighting that took place in the liberation of the Italian cities of La Spezia and Genoa in late April 1945.

Sometime during this final drive to force the Germans to surrender, Warrant Officer Johnson was wounded in action. His US Army discharge paper states he was hit in the upper back and shoulders by the enemy while fighting in Massa-Carrara, Italy. For wounds received in combat, Johnson was awarded the Purple Heart medal, becoming one of the few African-American soldiers to receive the esteemed decoration during WWII.

When the German forces in Italy finally surrendered on May 2, 1945, Johnson became part of the occupation forces. During his time in Italy, James met Luciana Scotti, a 21-year-old Italian woman. They quickly fell in love and married in Leghorn, Italy, on February 25, 1946. In January 1947, they had their first child, Maria, who was born at the US Army 61st Station Hospital in Ardenza Leghorn, Italy.

James returned to the states in June 1947 with his wife Luciana and daughter Maria. He had served honorably in foreign service for nearly three years. After returning to the states, he continued his military career, stationed at Camp Breckinridge, Kentucky, attached to the 101st Airborne Division. He served as 101st HQ Administrative Assistant and eventually became commanding officer of the 272nd Army Band. Johnson was honorably discharged from the US Army in 1952 as a Chief Warrant Officer.

Although the Allied forces were content with their rapid success in Italy, for James Johnson and the Buffalo Soldiers, it was a bittersweet victory as the Army considered the 92nd a complete failure. As the only all-black infantry division serving in combat during WWII, the division had been strictly watched and criticized. Racially motivated reports of poor performance, desertion, and low morale were submitted to the Army by senior white Army officers. But official records and numbers tell a different story. During their time in Italy, the 92nd advanced more than 3,000 square miles and captured more than 20,000 German prisoners. They also suffered heavy casualties – with more than a quarter of the unit killed or wounded in action. By war's end in 1945, the 92nd earned more than 12,000 decorations and citations – including two Medals of Honor.