Mencken was born in Baltimore, Maryland, on September 12, 1880. He was the son of Anna Margaret (Abhau) and August Mencken, Sr., a cigar factory owner. He was of German ancestry and spoke German in his childhood. When Henry was three, his family moved into a new home at 1524 Hollins Street facing Union Square park in the Union Square neighborhood of old West Baltimore. Apart from five years of married life, Mencken was to live in that house for the rest of his life.
In his best-selling memoir Happy Days, he described his childhood in Baltimore as "placid, secure, uneventful and happy."
When he was nine years old, he read Mark Twain's Huckleberry Finn, which he later described as "the most stupendous event in my life". He became determined to become a writer and read voraciously. In one winter while in high school he read Thackeray and then "proceeded backward to Addison, Steele, Pope, Swift, Johnson and the other magnificos of the Eighteenth century". He read the entire canon of Shakespeare and became an ardent fan of Kipling and Thomas Huxley. As a boy, Mencken also had practical interests, photography and chemistry in particular, and eventually had a home chemistry laboratory in which he performed experiments of his own devising, some of them inadvertently dangerous.
He began his primary education in the mid-1880s at Professor Knapp's School on the east side of Holliday Street between East Lexington and Fayette Streets, next to the Holliday Street Theatre and across from the newly constructed Baltimore City Hall. The site today is the War Memorial and City Hall Plaza laid out in 1926 in memory of World War I dead. At fifteen, in June 1896, he graduated as valedictorian from the Baltimore Polytechnic Institute, at the time a males-only mathematics, technical and science-oriented public high school.
He worked for three years in his father's cigar factory. He disliked the work, especially the sales aspect of it, and resolved to leave, with or without his father's blessing. In early 1898 he took a writing class at the Cosmopolitan University. This was to be the entirety of Mencken's formal education in journalism, or in any other subject. Upon his father's death a few days after Christmas in the same year, the business reverted to his uncle, and Mencken was free to pursue his career in journalism. He had applied in February 1899 to the Morning Herald newspaper (which became the Baltimore Morning Herald in 1900) and had been hired as a part-timer there, but still kept his position at the factory for a few months. In June he was hired as a full-time reporter.