Historically, Portland is one of the older towns of San Patricio County, and was laid out in 1890 by John G. Willacy, one of the original settlers who later became a State representative and who gave his name to Willacy County in the southern portion of Texas. 

The land was originally part of the Taft Ranch and was located on the 30- to 40-foot high bluff overlooking both the Corpus Christi Bay and the Nueces Bay, on the San Antonio and Aransas Pass Railroad line which was completed from San Antonio to Corpus Christi in 1886. The original townsite for Portland was comprised of 640 acres purchased by Mr. Willacy in November, 1890. On February 6, 1891, Willacy sold this land to the Portland Harbor and Improvement Company and, on June 19 of the same year, officers of the company filed an official map of the City at the San Patricio County courthouse. 

Early in its history, the community became a truck gardening center. Watermelons, cantaloupes, beans, turnips, radishes, tomatoes, beets, cabbage, lettuce, carrots, onions, sweet potatoes and Irish potatoes were among the principal crops. Truck gardening within this area declined rapidly after the Valley became a competitor. 

The New England Land & New England R.E. Company eventually purchased 1,280 acres of land east and northeast of the original townsite. Most of the stockholders of these companies resided in Portland, Maine; thus, the town was named in honor of their hometown. 

Over the years, the community continued to grow and thrive to its present size of approximately ten square miles of land area. Portland's population has grown from the original 192 persons in 1900 to an estimated 22,000 persons in 2021. The population nearly doubled between 2012 and 2021 due to the strategic location and location of several major industrial employers in the region.