The 9th Massachusetts Infantry was involved in the battle of Gaines Mill during which the regiment lost 57 killed, 149 wounded, and 25 were reported missing after the battle. According to another source, the regiment lost 82 men in the battle. It was here the regiment lost while attempting to defend a bridge while the remainder of the Union army was retreating. Just 4 days later, at Malvern Hill, the regiment suffered 166 casualties. At Gettysburg, the regiment, while defending Big Round Top, the regiment lost another 26 casualties. In May 1864 at the Wilderness, the 9th lost 26 killed, 108 wounded, and 3 reported missing. In the regiment's last battle, which was at the Salient at Spotsylvania Court House, the 9th lost 25 killed, 71 wounded, and 9 were reported missing.

In just 3 short weeks, the regiment was back in Boston in order to muster out of service. 

After leaving Boston in June 1861, the 9th Massachusetts Infantry regiment received a beautiful flag with funding provided by Tristram Campbell. Campbell, a son of Erin, who was born in 1828 in Culdoff Donegal Ireland, who had by 1862, made his way to  America, and had become an oil dealer. The flag was furnished by Thomas C. Savory of Boston, Massachusetts. The flag is described as a green silk flag bordered with gold fringe with various mottos painted on the front and back of the panels. "Erin go Bragh," "Duty to God and our country," "Faithful unto Death," Of course, the Irish harp and Irish wolf dog appear on the flag as well. On a reverse side of the flag were the words, "Presented to the 9th Regiment Massachusetts Volunteers, by their friends Sept. 1, 1862." The staff of the flag is described as being "handsomely carved, and surmounted with an Irish pike, a most formidable weapon." 

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