Grimwood's first attempt at a career in the SANFL was unsuccessful, he was on the books for Norwood in both 1965 and 1966 but didn't play a senior game. A rover, he got his second chance in 1971 when he was picked up by Port Adelaide with whom he would play 33 games over three years. In 1974 he crossed to West Adelaide and won their best and fairest award in 1976. His best season came the following season when he won the 1977 Magarey Medal by ten votes. Grimwood finished with 102 games for West Adelaide, he would have played more though had he not suffered from constant hamstring problems.
On November 28, 2011, Grimwood was sentenced to six years imprisonment with a non-parole period of two-and-a-half years for having sexual intercourse with a 14-year-old girl in 1985 and 1986.