Brock was born to Don and Lana Greenfield Don, a former teacher and current business owner, and Lana, a high school English teacher and business owner. Since January 2015 Brock and his mother, Lana, have served in the South Dakota Legislature as the only Mother/Son to ever serve simultaneously.
Greenfield graduated from Doland High School in 1994 and earned his Bachelor of Science in 1999 from Northern State University.
Brock played basketball and baseball while in school. Brock is in the Senate, and Lana is in the House, both representing District 2. In winning the seat, Greenfield became the youngest Senator in South Dakota history, a distinction he still holds.
2008 Term limited from remaining in the Senate, and with House District 6 incumbent Republican Representative Paul Nelson leaving the Legislature and leaving a House District 6 seat open, Greenfield ran, along with incumbent Kristi Noem, and the two were successful in winning House seats.
2010 When incumbent Republican Representative Kristi Noem ran for United States House of Representatives and left a District 6 seat open, Greenfield and Burt Tulson were unopposed for both the June 8, 2010 Republican Primary and the November 2, 2010 General election where Greenfield took the first seat with 4,991 votes (5349%) and Tulson took the second seat. 2012 Redistricted to District 2 with Republican Burt Tulson, and with incumbent Democratic Representatives Paul Dennert running for South Dakota Senate and Elaine Elliot leaving the Legislature leaving both District 2 seats open, Greenfield and Representative Tulson were unopposed for the June 5, 2012 Republican Primary.
In the four-way November 6, 2012 General election Greenfield took the first seat with 6,049 votes (335%) and Representative Tulson took the second seat ahead of Democratic nominees Dennis Nemmers (who had run for a legislative seat in 2008 before withdrawing) and Danny Miles.
2000 When incumbent Republican Senator Randall Frederick left the Legislature and left the Senate District 6 seat open, Greenfield was unopposed for the June 6, 2000 Republican Primary and won the November 7, 2000 General election with 5,016 votes (511%) against Democratic Representative Doug Kazmerzak. 2002 Greenfield was unopposed for the June 4, 2002 Republican Primary and won the November 5, 2002 General election with 5,731 votes (558%) against Democratic Representative Charlie Flowers. 2004 Greenfield was unopposed for the June 1, 2004 Republican Primary and won the November 2, 2004 General election with 7,268 votes (691%) against Democratic nominee Ronald Larson. 2006 Greenfield was challenged by Representative James Holbeck in the June 6, 2006 Republican Primary. Greenfield placed first with 1,769 votes (708%). And won the November 7, 2006 General election with 5,515 votes (606%) against Democratic nominee Ron Foster who had run for a House seat in 2004.
Greenfield was consecutively a member of the House for District 6 from January 2009 until January 11, 2013, and a member of the South Dakota Senate for District 6 from January 2001 until January 2009.
