Smith was born into a family of sharecroppers in 1912 in rural Williamson County.  Like many farm families of the time, it was a large household, with thirteen brothers and sisters in all.  Eventually, the Smiths settled near the West Texas community of Lamesa, where Smith graduated high school in 1928.

After graduation, Smith moved to Lubbock where he enrolled at Texas Tech, working his way through college.  He graduated with a business degree and began a small chain of movie theaters in the city by the 1940s.  In 1944, he was elected to the state house of representatives, the first of three terms.

Smith took a few years off from politics to tend to his business interests in Lubbock and take care of his growing family.  In 1956, he returned to politics with a victory in the election for state senate.  In 1962, he was elected lieutenant governor by a wide margin.

When President John F. Kennedy was murdered in Dallas on November 22, 1963, Gov. John Connally was also seriously wounded in the attack.  His critical injuries forced Smith to assume the role of acting governor in the months it took Connally to recover.  Smith was praised for his steadiness in the face of the assassination.

In 1968, Connally declined to seek a fourth term as governor, leaving an opening for Smith.  He jumped into a wide field for the Democratic Primary in 1968.  In one appeal to voters, he sent out letters to the 47,000 families named Smith in the state, asking, “Don’t you think it’s about time one of us was governor?”  Smith ended up winning the primary and swept to a win in the general election with 57% of the vote.

Smith was the first governor from West Texas.  He secured some improvements for education, such as a plan to phase in a raise for teachers over a ten year period as well as increased funding for vocational schools.  He won approval of a state minimum wage law and helped secure new medical schools in Houston and Lubbock.

Smith was described as an affable and grandfatherly figure which won him a lot of admirers for his straightforward and open approach to governing.  However, his public relations efforts were sometimes clumsy, leading detractors to call him “Pop,” for “Poor Old Preston.”

After his second term began in 1971, the state government was embroiled in what became known as the Sharpstown Scandal.  A Houston businessman named Frank Sharp had tried to push through favorable anti-regulatory legislation for his bank and insurance company, giving certain legislators stock in his companies.  House Speaker Gus Mutscher, who had engineered the legislation, ended up being indicted and convicted in the scandal though that conviction was later overturned.

Though Smith was never implicated in any wrongdoing, the scandal undermined faith in his administration.  He ran for a third term in 1972 as calls for reform rose.  He was humiliated with a fourth-place finish in the primary, far behind South Texas businessman and rancher Dolph Briscoe.

He attempted a political comeback in 1978, running for governor again, but he placed third in the primary.  He remained active in Lubbock and worked as a lobbyist for Texas Tech for many years.  He served as chairman of the state College and University Coordinating Board in the early 1980s.

His health declined following a car accident in 2003.  He died in October of that year at the age of 91.  He was widely praised by both Democrats and Republicans for his years of dedicated service to the state.   The next year, his hometown of Lubbock honored his memory by renaming the airport after him.