A Roman Catholic,Weichmann entered St. Charles College when he was sixteen with the intention of becoming a priest. While at the college in Maryland he met John Surratt. Both men decided to abandon their plans of entering the church and moved to Washington where Weichmann found work as a schoolteacher.
Soon after the start of the American Civil War Weichmann went to work as a clerk in the rapidly expanding War Department. In November, 1864, Weichmann became a lodger at the boarding house owned by Mary Surratt, the mother of John Surratt. This brought Weichmann into contact with other friends of the family including John Wilkes Booth.
After the assassination of Abraham Lincoln, Weichmann, as an associate of John Wilkes Booth, was arrested and threatened with being charged with the crime. Later it was claimed that Weichmann was offered a deal and that in return for his testimony in court, he would be allowed to go free.
At the trial John M. Lloyd told the court that on the Tuesday before the assassination Weichmann and Mary Surratt visited him. Lloyd claimed that Mrs. Surratt "told me to have those shooting-irons ready that night, there would be some parties who would call for them. She gave me something wrapped in a piece of paper, which I took up stairs, and found to be a field-glass. She told me to get two bottles of whisky ready, and that these things were to be called for that night."
When Weichmann testified he claimed he could not hear what Mary Surratt said to John M. Lloyd as they spoke in hushed tones. He did tell the court that he had seen John Wilkes Booth, Lewis Powell, George Atzerodt and David Herold in Mrs. Surratt's house together. This supported the prosecution's claim that the boarding house was where the assassination plot had been planned.
Weichmann also testified that he was with John Wilkes Booth in Washington on 23rd December when he had met Samuel Mudd, another man charged with conspiring to assassinate Abraham Lincoln. This was important evidence as Mudd denied that he had met Booth in Washington.
On 29th June, 1865, Mary Surratt, Lewis Powell, George Atzerodt and David Herold were also found guilty of the conspiracy to murder Abraham Lincoln and were hanged at Washington Penitentiary eight days later. It has been argued by historians that Weichmann's testimony in court had been crucial in the conviction of Mrs. Surrett.
After the trial Edwin M. Stanton, the Secretary of War, and Joseph Holt, the government prosecutor, helped get Weichmann the post as clerk of the Philadelphia Custom House. Weichmann lost the job in November, 1866, when President Andrew Johnson decided to purge people that held jobs obtained via the Republican Party.
When Ulysses Grant became president, Edwin M. Stanton arranged for Weichmann to get his job back with the Philadelphia Custom House. When Grover Cleveland and the Democratic Party came to power in 1886, Weichmann was sacked again. He now moved to Indiana where he established the Anderson Business College. Louis Weichmann died in 1902.