The school began in 1874 as Portland Seminary and sat on a 1-acre (4,000 m2) plot of land donated by J.C. Buntin, the son of the town's founder.

By 1956, the school's teaching staff had undergone a major change, with almost 100% of the teachers holding college degrees.

In the fall of 1960, an alumnus, William Coker, became principal and served until the beginning of the school year in 1974.

This position became full-time in 1973 with the hire of Mr. John Meece, who served as vice principal until 1984 when he assumed the principalship The year 1961 saw the last graduating class from Sumner County High School.

When William Coker retired from the principalship in 1974, John Meece became interim principal for several months.

In 1976, a new Sumner County Vocational Center was begun at an adjacent building to the high school.

Harvey Foster took over the administration of the schools in 1978 and served until 1984 when John Meece was appointed principal.

Under Mr. Meece's direction, the school expanded its physical facilities with the addition of the new wing and the renovation of the library and office space in 1986.

After Mr. Meece moved to the central office in the spring of 1994, Calvin Short became principal in the fall of 1994.

The Sumner County Vocational Center officially became a part of Portland High School.

Before his retirement in November 1998, Mr. Short oversaw the planning and building of the new school located at 600 College Street.