At the 1970 general election, he stood in the marginal Scottish constituency of Aberdeen South, and ousted the sitting Labour MP, Donald Dewar. He was re-elected there at three further elections, until the 1983 general election when he moved to contest Roxburgh and Berwickshire believing that this was a "safer" seat. However, Aberdeen South was held by the Conservatives, while Roxburgh and Berwickshire fell to the Liberal candidate Archy Kirkwood.
Sproat returned to Parliament nine years later, moving to England and succeeding Sir Julian Ridsdale as MP for Harwich in the 1992 general election. At the 1997 general election, he was defeated by the Labour candidate Ivan Henderson. Sproat stood again in Harwich at the 2001 election, but Henderson was returned with an increased majority.