The California Department of Public Works assigned 112 as the preliminary number of the future Interstate at Tracy as part of a route map submitted to the American Association of State Highway Officials (AASHO) on April 1, 1958. The route map utilized numbers in the first lower number block, avoiding 101 because of U.S. 101 and 104, 107, 108, 111 and 118 because those were existing state routes. Subsequent correspondence between AASHO and California on July 2 and 17, 1958 updated the route numbering plan, with 205 assigned to the route in Tracy because it connected I-5 with Interstate 5W. I-205 was approved as part of the statewide plan on November 10, 1958. Interstate 5W was eventually renumbered as part of I-580 in 1964.
Due to its role as a commuter route, the eastern end of Interstate 205 was widened from four to six lanes in 2007/08. I-205 expansion followed the four lane expansion of the freeway along SR 120 across Manteca, which accounted for the increase in traffic demands.