Dr. Wallace was indentured by his uncles Thomas and Michael Wallace, merchants of Glasgow, as his guardians, to Dr. Gustavus Brown, March 18, 1734, for the space of six years, to learn "Physick, Surgery, and Pharmacy." Michael Wallace arrived with Dr. Brown in America in June of 1734.

Whether Dr. Wallace first practiced medicine in Maryland is not known. He settled at Falmouth, Virginia by 1746. His practice extended throughout all that region of Virginia, even to the Blue Ridge in Culpeper, Fauquier and Loudoun Counties. The vestry of St. Mark's parish, Culpeper County, paid him 800 pounds tobacco, the then Virginia currency, for curing Eliza Maddox. On June 27, 1748 he purchased from William and Margaret Waugh, a large tract of land in Stafford County which he called "Ellerslie", after the family estate in Renfrewshire, Scotland.

Dr. Wallace, like most of Dr. Brown's medical students, was attracted by the virtues of one of the "nine Miss Browns;" and doubtless meeting with somewhat the same opposition from Dr. Brown which others had encountered, he determined to "run away" with his bride. She eloped from her father's house at night through a second story window, while Dr. Brown's medical students held the ladder and awaited for her below. They now rest side by side in the old colonial grave yard at Falmouth, Virginia.

Dr. Wallace’s half brother-in-law, Dr. Gustavus Brown, was one of the three attending physicians at President George Washington’s bedside at the time of his death.

The death of Dr. Wallace was caused by exposure to rain and snow during a professional visit to Culpeper County. He was thoroughly chilled, his clothing freezing stiff on his body. A severe attack of dropsy ensued, resulting fatally.