1800 - Arapaho Indians move into the area from further north, often hunting and camping in the St. Vrain Valley.
1820 - Major Stephen H. Long leads an expedition for the U. S. Topographical Engineers along the South Platte, draws first maps of region.
1838 - Bent, St. Vrain & Co. build a trading fort near where a creek or small river flows into the South Platte. Trappers begin to call it St. Vrain's creek.
1860 - Alonzo N. Allen moves his partially completed cabin from east of Boulder to the St. Vrain valley, near the confluence of St. Vrain and Left Hand creeks.
1861-70 Town of Burlington grows up around the Burlington Stage stop, where the Overland Stage crosses the St. Vrain. (Overland Stage route is modern U. S. 287, Main Street in Longmont).
1870 - The Denver Pacific and the Kansas Pacific railroads reach Denver, making travel to Colorado much easier.
1871 - The Chicago-Colorado Colony chooses a location, plats the original square mile and settles their new town, naming it Longmont. Residents of Burlington move themselves and most of the buildings to Longmont, about a quarter mile north.
1873 - The Colorado Central Railroad begins service from Golden to Longmont.
1876 - Colorado becomes a state.
1879 - Fire destroys the east side of the 300 block of Main Street in Longmont. The downtown is rebuilt in brick.
1887 - John H. Empson opens the J. H. Empson Cannery in Longmont, canning peas and other vegetables.
1898 - J. C. Penney opens a meat market on Main Street. After failing in this business, he worked for T. M. Callahan's Golden Rule Store in Longmont. Ultimately, Penney would set up a Golden Rule Store in Kemmerer, Wyoming that would be the beginning of his retail empire.
1899 - The first official Pumpkin Pie Days for which Longmont became famous, baking 10,000 pies for people coming from all across the Front Range. The original Pumpkin Pie Days were held from 1899-1915.
1900 - Theodore Roosevelt makes a campaign stop in Longmont.
1903 - The Longmont Sugar Factory built. Soon renamed the Great Western Sugar factory, it played a vital role in farming and the development of Longmont.
1905 - Masonic Temple on Main Street burns down, but the Dickens Opera House next door is saved. The Masonic Temple would be rebuilt.
1908 - Longmont High School "Beetdiggers" became the "World Champion Interscholastic Football Team" beating Chicago's Englewood High in a Christmas Day game in Denver.
1911 - Longmont builds a hydropower plant on the North St. Vrain river, providing reliable electric power to its residents.
1913 - The Carnegie library in Longmont opens at 4th Ave. and Kimbark St. Also, the greatest snow in the history of the area occurs.
1918 - Great influenza epidemic forces City Council to ban public gatherings, but many Longmont residents die in spite of precautions.
1923 - The Ku Klux Klan, riding a wave of statewide popularity, elects a majority in the Longmont City Council. The Council began ousting longtime City officials and replacing them with Klan members.
1926 - City Council, controlled by the Klan, votes to build Chimney Rock Dam, a combination water storage and hydropower project, located just above the existing Longmont Dam on the North St. Vrain. Cost estimates rise from $85,000 to $350,000.
1927 - All Klan candidates for City Council are defeated in the April election, and construction is halted on Chimney Rock Dam.
1930s - Many area farms are lost during the Great Depression.
1941-1945 Longmont citizens serve around the world during World War II. In Longmont, the Great Western Sugar dormitory at 3rd Ave. and Kimbark St. is converted to house Italian and German POWs who then worked on area farms.
1946 - Gibson Manufacturing Co. opens a tractor factory in Longmont, the first new heavy industry to locate in Longmont in over forty years. Production of Gibson tractors ceases, however, in 1952.
1950 - Longmont's population is 8,099.
1955 - A United Airlines DC-6B airplane crashes eight miles northeast of Longmont, killing all 44 aboard. Subsequent investigations showed that it was brought down by a bomb in a suitcase.
1960 - Longmont's population is 11,489.
1961 - City Charter adopted and Longmont becomes a home-rule city.
1962 - The Federal Aviation Administration opens an air traffic control center in northwest Longmont.
1965 - IBM announces construction of a large plant between Boulder and Longmont, accelerating the pace of growth and the shift of the economic base from agriculture to high technology.
1970 - Longmont's population is 23,209, double that of ten years earlier. The Kuner-Empson cannery closes.
1972-5 "Project 75" complex at 3rd Ave and Kimbark Street opens, including a new library, Civic Center and police station.
1975 - Longmont native Vance Brand flies on the Apollo-Soyuz mission which linked Soviet and American spacecraft.
1977 - Closing of the Great Western Sugar Factory in Longmont.
1980 - Longmont's population is 42,942, almost double that of ten years earlier. On August 14, Longmont police officer Glenn Herner shoots and kills two men, Juan Garcia and Jeff Cordova, during an altercation after a traffic stop. Herner was later acquitted of manslaughter charges. The aftermath of this tragedy led to massive changes in police training and to the founding of El Comite.
1986 - Twin Peaks Mall opens.
1990 - Longmont's population is 51,524.
1993 - Construction of a new Library and a new Safety & Justice Center.
2000 - Longmont's population is 71,093.
2002 - Construction of a new Recreation Center, a Museum & Cultural Center, and improvements to Roosevelt Park completed.
2010 - Longmont's population is 86,270
2011 - Closing of the Butterball turkey processing plant, the last major agricultural industry in Longmont.
2013 - A major flood damages homes, businesses, and roads in Longmont and across the Front Range.
2015 - Village at the Peaks dining and shopping area opens, replacing Twin Peaks Mall.