- 1857 Milton County was created.
- 1858 Alpharetta is incorporated as the Milton County seat.
- 1863-1865 Alpharetta suffers smallpox epidemic.
- 1871 Alpharetta Male and Female Academy opens.
- 1870s Milton County builds its second courthouse on the same site as the current city hall.
- 1907 The Webb Building is built on the site of the old Harris Hotel, where the current downtown city park is now located.
- 1921 The last day of school in the Alpharetta Male and Female Academy is Dec. 21.
- 1922 The new Milton County School opens on January 2 with grades one through eleven.
- 1930 Alpharetta installs its first central water system.
- 1932 Milton County, nearly bankrupt, merges with Fulton County on January 1.
- 1934-1935 The Log Cabin is built on the old Milton High School campus by the students belonging to the Future Farmers of America organization.
- 1940s The primary means of support for Alpharetta residents become poultry, livestock, and truck farming. The opening of Lockheed and the General Motors Assembly plant in Doraville creates jobs.
- 1948 The Alpha Theatre opens on Milton Avenue. The movie house closed around 1960 as television and drive-in theater became more popular.
- 1959 The Bailey Johnson School for African American students opens on Kimball Bridge Road.
- 1955 The old county courthouse is demolished.
- 1957 A new municipal building containing city hall opens on the former courthouse site.
- 1974 The initial section of Georgia 400 north of I-285 was officially dedicated on May 24.
- 1980s Mobil Land Development Company begins construction in Windward, an exclusive planned subdivision and office park on 3,400 acres of former farmland on the eastern side of Georgia 400.
- 1993 North Point Mall opens with anchors Rich's, Lord and Taylor, Mervyn's, JC Penney and Sears.
- 2005 The old Milton High School closes after 84 years. Students are relocated to a new campus, also called Milton High School on Birmingham Highway in what is now the City of Milton.