Sampson began his career as a schoolteacher and principal in Glencoe, MN. A short time later he entered the law office of Judge Atwater in Minneapolis. In 1877, he was admitted to the bar and began private practice. In 1883, he joined Judge Seagrave Smith to form the highly successful law firm of Smith & Reed. Sampson specialized in land title and real estate law.

"He was descended from one of the old New England families, his ancestors having settled in Oxford county, Maine, after the Revolutionary war. Among his ancestors was Governor Bradford of the Plymouth colony, and in the maternal line Sampson A. Reed was also descended from ancestors who came to America on the Mayflower. It was not long after the Revolutionary war that many soldiers made their way to the wilds of Maine and there founded homes, which number included representatives of the Reed family, this including one, Sampson Reed, whose father had died in Massachusetts and who in 1795 went with his mother to Hartford, Maine, then a wild and undeveloped district. The name, 'Sampson,' has been carried down through successive generations, Sampson A. Reed of this review being the fourth to bear that name." --History of Minneapolis, Gateway to the Northwest

"He was recognized as one of the strong and able members of the Minneapolis bar, as a man of high integrity and of wide popularity because of his sterling characteristics, his geniality and cordial disposition. While fourteen years have passed since he was called to his final rest, his memory is yet cherished by many who were his close associates in life." --History of Minneapolis, Gateway to the Northwest