The origins of Three Portlands are unknown, and the subject of much speculation, but it is generally assumed that the city did not exist prior to the incorporation of Portland, Oregon in 1851. Parahistorian L. Rowe has theorized that Three Portlands may have been formed by the global thaumic backlash from the 6th Occult War, which would place the city's origin in the 1880s. The earliest reliable record of Three Portlands is an internal memorandum of the British Occult Service dating from 1899, documenting the activities of various minor occult cabals in the south of England and the adjacent multiverse; this memo indicates that Three Portlands had been lightly populated for at least a decade at that point, which closely matches Rowe's timeline.

In 1905, following negotiations with the native genius loci, the BOS began using Three Portlands to house Sidhe refugees from the ongoing collapse of Avalon. Although many of these would eventually be relocated to isolated portions of the Hebrides, the influx of displaced Sidhe greatly increased the city's population and left a lasting influence on its culture and future development. However, aside from the Sidhe relocation efforts, British authorities remained largely disinterested in Three Portlands, preferring to focus their attentions on occult activities on Earth.

With the start of the First World War, the BOS became committed to preventing an outbreak of occult warfare; most British operatives in the city were recalled, leaving Three Portlands to manage its own affairs. In 1915, a city council was elected and a municipal government organized, although this went unrecognized by British officials, who continued their policy of treating the city as the demesne of the native genius loci; this lasted until 1922, when the city charter was amended to include the genius loci in the government as the Mayor.

The near total absence of any paranormal authorities was quickly capitalized upon by various criminal organizations, who used the city's extensive Way network to evade US customs authorities and — following the passage of the Volstead Act — Prohibition agents. Even after the end of the World War, the BOS did little to curb the growth of paracrime, despite the increasing British presence within the city. However, recognizing the strategic importance of the city's Way network, British authorities began working to actively conceal its existence from other normalcy preservation agencies, including those of the United States government.

Despite the efforts of their British counterparts, American law enforcement discovered Three Portlands in 1929, after Bureau of Prohibition agents managed to track the operations of the bootlegging group known as the Chicago Spirit to the city. In accordance with federal policy at the time, the existence of the city was concealed, and all related cases were transferred to the Bureau of Investigation. BOI Director J. Edgar Hoover immediately began waging a clandestine war against the Chicago Spirit within Three Portlands. While early raids and sting operations were highly successful, the BOI soon met opposition from the citizens and city government, who resented the unprecedented intrusion upon their autonomy, ultimately culminating in a firefight between BOI agents and members of the city council.

In response, Director Hoover had all known entrances to Three Portlands within the United States placed under siege, imprisoning anyone who attempted to enter or exit the city. This standoff lasted for almost two months, only ending with the intervention of President Herbert Hoover and the formulation of the Hoover Mandate. Under the Hoover Mandate, the United States would guarantee the autonomy of Three Portlands; in return, federal agents would be allowed to operate within the city to a limited degree. Fearing furthering escalation, the city council eventually accepted President Hoover's terms, despite protests from many of the city's inhabitants.

The British response to the Hoover Mandate was extremely negative, as the United States' guarantee of autonomy was perceived as a threat to their expanding operations in the city, particularly their use of the Way network. Although direct confrontations between British and American agents were rare, tensions remained high throughout the next decade, as both sides made competing claims of jurisdiction and sought to exercise exclusive authority within Three Portlands. The outbreak of the 7th Occult War in 1939 finally encouraged the British Occult Service to seek a rapprochement with the Unusual Incidents Unit, eventually leading to the Howard-Grant Agreement in 1940, under which the United Kingdom abandoned its claims on Three Portlands in exchange for a number of wartime concessions.

During the 7th Occult War, Three Portlands played a significant role as a clandestine supply route and shelter for the Allied Occult Initiative , with the AOI making extensive use of the city's Way network to move personnel and materiel between Britain and North America. As a result, in 1943, Three Portlands came under attack by a force of Obskuracorps commandos and Thule battlemages , who apportated onto the Isle of Portland and opened a Way into the city, with the intention of seizing the ICSUT campus and looting the AOI storehouse there. Despite the unexpectedness of the attack, a combined force of UIU agents and BOS operatives, alongside ICSUT faculty and students, municipal police golems, and local citizens, succeeded in repelling the assault.

The city grew rapidly in the post-war era, driven by a global tightening of the Veil, which led to the mass expulsion of parahumans, occultists, and anartists from urban areas in Europe and North America, many of whom sought refuge in paranormal enclaves such as Three Portlands and Backdoor Soho. The city's status as a major paranormal freeport was further cemented in 1956, when Prometheus Laboratories chose to massively expand its operations in Three Portlands, motivated by its unique taxation laws and its exemption from most Veil regulations. Over the next few decades, Three Portlands evolved to become a hub of both anartistic endeavors and parascientific research, reflecting many of the same cultural pressures that were shaping the adjacent Portland, Oregon.

As part of the Howard-Grant Agreement, the UIU and the BOS had both agreed to continue the policy of concealing Three Portlands from most other normalcy preservation agencies, with exceptions made during the 7th Occult War to facilitate AOI operations. While these efforts were never totally effective, especially given widespread awareness of the city among the paranormal communities in adjacent regions, they were deemed necessary to preserve the city's status as a paranormal freeport. Following the reorganization of the AOI into the Global Occult Coalition, the primary focus of these efforts then became concealing the city from the Foundation, which was — and still is — viewed as the single largest threat to Three Portlands' autonomy.

Although the Foundation's eventual discovery of the city was largely viewed as inevitable, UIU agents, relying heavily on memetic misinformation campaigns, succeeded in delaying this event until 1970. This marked the start of a major shift in relations between the UIU and Three Portlands residents, ushering in an improvement of public opinion that swelled even further following the death of Director Hoover in 1972. Local recruiting into the UIU increased dramatically, and by the end of the decade, almost a quarter of the Unit's irregular agents were from Three Portlands.

Starting in the late 1970s, Three Portlands emerged as a center for the nascent Maxwellist movement; the city's thriving paratech industry and relaxed Veil made it a perfect fit for the transhumanist religion, which was facing active suppression efforts by major normalcy organizations on Earth. As Maxwellism continued to spread throughout the city and its paratech industry during the 1980s, the GOC responded by orchestrating the creation of Eurtec, a paranormal freeport controlled by one of its member organizations, with the intention of establishing a competing hub of parascientific research under its own oversight. Under the GOC's sponsorship, Eurtec rapidly grew to rival Three Portlands, leading to a gradual erosion of the city's paratech industry.

The decline of the city's paratech industry was further exacerbated by the economic shock from the end of the Cold War and its accompanying anomalous arms race. While Eurtec offered generous bailouts to local businesses, Three Portlands' paratech companies were left to weather the downturn on their own, resulting in the near total collapse of the city's paratech industry, culminating in the closure of the Three Portlands offices of Prometheus Labs. At the same time, organized paracrime was beginning to return to the city, this time in the form of the Lighthouse Mafia.

The city finally began to recover with the start of the new millennium, due in large part to the efforts of Portland native Vincent Anderson and his paratech startup, Anderson Robotics. Taking advantage of the void left by Prometheus Labs, Anderson established his corporate headquarters in Three Portlands and rapidly expanded, spurring a revival of the city's local industry. This, along with his numerous philanthropic contributions throughout the city, helped endear Anderson to Portlands' population, particularly its large Maxwellist contingent.

This popular support made it politically unfeasible for the UIU to take action against Anderson, a fact which he exploited to circumvent — and eventually ignore outright — federal and international restrictions on paratech development. This flagrant disregard for normalcy eventually drew the attention of the Foundation, which launched a protracted campaign against Anderson and his operations, with limited success. Anderson further consolidated his company in Three Portlands, using the Hoover Mandate to shield himself from Foundation reprisals.

The stalemate between Anderson Robotics, the UIU, and the Foundation lasted until 2018, when it was discovered that Anderson had used his Saker androids to infiltrate the United States Government. A warrant for his arrest was issued and he vanished from public life — both the Foundation and the UIU believed he had been taken into Foundation custody. However, Anderson Robotics continued operating with Anderson secretly in control, until his final capture in 2024 by a joint task force of the Foundation and the UIU. The accompanying raid on Anderson Robotics' Headquarters in downtown Three Portlands was the largest concerted UIU action in the city since Hoover's Siege, leading to widespread protests and a decline in public opinion of the Bureau to the lowest levels of the post-Hoover era.