FM 1623 was first designated in Blanco County in 1951, and ran from US 281 in Blanco to the west approximately. Its length was extended to 10.3 mi in 1953 and increased again in 1954; the designation ended at the GillespieCounty line. The extension into Gillespie County and to US 290 would be approved in 1955. The route was redesignated in 1969 as an RM route, the same year the extension to RM 2721 was added.