Thomas Greer Mackenzie was born on September 29, 1839, to John P. Mackenzie and Theresa Carrere. He was probably born in Baltimore, Maryland, and was christened on July 12, 1841, in the city’s Associate Reformed Church. Mackenzie came from a long line of physicians, including his father, grandfather, and several uncles. “In the science of medicine,” one biographer wrote, “the Mackenzie name is peculiarly distinguished at home and abroad, and stands unsurpassed in the medical annals of Baltimore.” Thomas and several of his brothers followed their father into the medical profession. In 1856, he enrolled at the University of Virginia, where he studied chemistry, natural philosophy, Latin, Greek, and modern languages. He transferred to the University of Maryland after 1858, and he graduated from there in 1861.

Mackenzie staunchly supported the Union, joining hundreds of Baltimore citizens in calling for a pro-Union meeting to be held on January, 10 1861. He received a commission as an assistant surgeon in the Regular Army on April 16, 1862, and mustered into service on August 1. He primarily served at hospitals in the Washington, D.C. area, including at Slough General Hospital and the Kalorama General Hospital. In September 1863, he held the rank of first lieutenant. He later received a promotion to captain, and on March 13, 1865, he was brevetted a major for faithful and meritorious service during the Civil War.

While in the service, he married Laura Owings on February 17, 1863, in Frederick, Maryland. Laura gave birth to a daughter, Theresa, on December 11, 1864, in Alexandria, Virginia. On December 28, 1866, Mackenzie contracted a lung infection and died of pneumonia four days later on New Year's Day 1867 at Kalorama Hospital. His widow successfully applied for a pension of $20 per month on June 1, 1867. She received back pay from the time of her husband’s death and obtained at least one pension increase before her death on September 16, 1895.