Pacific Northern Gas Ltd. was incorporated under the laws of British Columbia as a private company on October 28, 1965 and was converted to a public company on November 4, 1968.

In 1968 PNG constructed a transmission line from a point 50 kilometers north of Prince George at Summit Lake on the Spectra Energy  pipeline system west through to Prince Rupert and Kitimat on the British Columbia coast. Gas transmission service commenced in 1969, with service provided mainly to large industrial customers. Following completion of construction of the gas transmission line, PNG developed gas distribution services in the communities adjacent to its transmission line.

In 1993 PNG acquired Northland Utilities (B.C.) Limited. This system serves the Dawson Creek and Tumbler Ridge areas and is connected to the Spectra Energy pipeline system.

In 1997 PNG completed three more acquisitions; the Centra Gas Fort St. John Inc. system which added over 8,000 accounts in the Fort St. John area to the customer base, the Granisle grid, a small propane vapour distribution system located adjacent to PNG's western service area, and Peace River Transmission Company Limited which owned the transmission pipeline used to transport gas between Spectra Energy pipeline facilities and the Dawson Creek distribution system.

The Fort St. John, Dawson Creek and Tumbler Ridge gas pipeline systems are owned and operated by PNG's wholly owned subsidiary, Pacific Northern Gas (N.E.) Ltd.

In 2000, a major company reorganization was undertaken in response to liquidity issues created when PNG's largest customer, Methanex Corporation, shut down for a one-year period. A Customer Care Center was opened in Terrace, thereby permitting closure of ten community service offices and significant reductions in labour requirements and operating costs. Additional savings were realized through downsizing and consolidation of responsibilities of operations and maintenance personnel.

On December 18, 2003 Tricor Acquisition (STP) Inc., a subsidiary of Tricor Pacific Capital, Inc. acquired PNG's Class A non-voting and the Class B voting common shares held by Westcoast Energy Inc. In connection with the acquisition, all of PNG's Class A and Class B common shares were reclassified as one class of common shares with one vote per share. As a result of the acquisition, Tricor owned 37 percent of the outstanding common shares of PNG. On April 12, 2005, all of the common shares of PNG owned by Tricor were sold through a secondary public offering at a price of $19.40 per common share, for total gross proceeds of approximately $26 million.

On August 30, 2005, Methanex Corporation gave notice of termination of its transportation agreement with PNG. Under the terms of the agreement, Methanex made a termination payment to PNG of approximately $23.3 million on February 28, 2006, the effective date of the termination. On August 16, 2006 the B.C. Utilities Commission approved PNG's proposal to record the termination payment in an interest bearing credit deferral account, to be amortized into income over the 44-month period from March 1, 2006 to October 31, 2009, the original date of expiry of the transportation agreement. At the same time, the Commission approved PNG's application to recover in customer rates the reduction in revenue from the Methanex contract termination.

On July 17, 2006, PNG entered into an agreement with Galveston LNG Inc. (the parent company of Kitimat LNG Inc.) to form Pacific Trail Pipelines Limited Partnership (PTP). PTP, owned equally by PNG and Galveston, enables PNG to pursue environmental certification of the facilities required to transport natural gas from the proposed Kitimat LNG regasification terminal near Kitimat, to the Spectra Energy pipeline system at Summit Lake, near Prince George (the KSL Project). Galveston LNG is endeavouring to secure LNG supply for the Kitimat LNG terminal, one of several conditions to be met before PTP can proceed with construction of the KSL Project.

On December 20, 2011, PNG was acquired by AltaGas Ltd. of Calgary, Alberta. AltaGas is an energy infrastructure business with a focus on natural gas, power and regulated utilities.