Weichmann was born in Baltimore, the son of German immigrants. The family surname was originally Wiechmann, but as in the case of many who emigrated to the United States, the name underwent several phonetic spelling changes. His father Johann was a Lutheran, and his mother Maria was a Catholic. Johann Weichmann was a tailor by trade, and he moved with his wife and their five children first from the vicinity of Baltimore to Washington D.C., and later to Philadelphia, Pennsylvania. Louis claims to have attended Central High School in the autobiographical work mentioned below. Central High School records show Louis Weichmann was a member of the 36th graduating class. He was admitted July 1856 and left in Sept. 1858 with a Partial Course Diploma. He wrote in his autobiographical work, "A True History of the Assassination of Abraham Lincoln and of the Conspiracy of 1865," that he desired to pursue a career as a pharmacist, but, at the behest of his mother, he reluctantly agreed to study for the Roman Catholic priesthood. At the age of seventeen, he entered the seminary at St. Charles College in Ellicott City, Maryland. There, he met and befriended a fellow seminarian, John Surratt. This friendship was to later have profound consequences for both of them.