rm 620 was originally designated on july 9, 1945 as farm to market road 620 from round rock to sh 29, the former designation of us 183 south of liberty hill until may 23, 1951. on may 13, 1946, the road was extended to the travis county line. on august 26, 1948, the road was extended to the former community of hickmuntown, also called four points, at the location of the current intersection with rm 2222. the road was completed on december 17, 1952 when it was extended to sh 71, known locally before august 31, 1965 as rm 93 (but was signed as sh 71 since october 31, 1955). the road received its ranch to market designation on october 1, 1956, and its urban road designation on june 27, 1995. the sh 45 toll road running along a portion of rm 620 opened in 2006. on february 28, 2013, the section from ih-35 to bi 35-l was cancelled and given to the city of round rock, along with part of bi 35-l itself.