Michael H. McGrath was born on December 9, 1817, in New York. He married Lucinda Cady around 1840, and they had at least three children: Sophia, born around 1841; Henry, born around 1843; and Emily, born around 1849. The family moved to Erwin, New York, around 1850, and McGrath worked as a carpenter there. In 1850, he owned $600 of real estate. He enlisted in the Union army on July 6, 1862, and mustered in as a 1st lieutenant in Company F of the 50th New York Engineers. He was promoted to captain on October 14, 1862. The regiment took part in the Battle of Fredericksburg, the Battle of Gettysburg, the Battle of the Wilderness, the Battle of Spotsylvania Court House, and the Siege of Petersburg. He mustered out in Fort Barry, Virginia, on June 13, 1865.
McGrath returned to Erwin after the war. The family moved to Elmira, New York, sometime before 1880, and McGrath worked as an architect there. He applied for a federal pension in April 1887 and secured one soon afterwards. He died in Elmira on March 18, 1896.