Preston was a non-commissioned officer with the Royal Canadian Engineers for a number of years. He also worked in sales and management insurance, and founded Preston House in 1984 as a group home for boys aged twelve to eighteen.
He was elected to the Ontario legislature in the 1995 provincial election, defeating Liberal incumbent Ron Eddy by about 3,500 votes in the mostly rural southern Ontario riding of Brant—Haldimand. The seat was formerly known as a Liberal stronghold, and Preston's victory was regarded by many as an upset. He served as a backbench member of Mike Harris's government for the next four years.
Preston's riding was eliminated by redistribution prior to the 1999 provincial election, and he chose to seek re-election in the urban riding of Hamilton East (which had not returned a Progressive Conservative MPP since 1955). He finished a distant second against Liberal Dominic Agostino, and has not sought a return to politics since then.