He finished his studies in Chicago, Illinois under Ebenezer Peek, subsequently one of the Judges of the Court of Claims, and was admitted to the bar in 1853.

He did not engage in politics except to show his colors in private conversation and at the polls; but, having known President Abraham Lincoln for a number of years, he was appointed by him in 1861 as the United States Attorney for the Territory of Washington.

McGilvra served one term in the Territorial Legislature of 1866–67, and during the session procured an appropriation of US$2,500 for a wagon road across the Cascade Mountains through the Snoqualmie Pass.

Subsequently they carried another branch of road up the Cedar River to the Black Diamond and Franklin coal mines.

That enterprise, which was undertaken at a critical time in the history of Seattle, had the effect to stay the confidence of the citizens, and assisted materially in building up the town in spite of all opposition, and the discrimination against it by the Northern Pacific Railroad.

Funds were raised and Judge McGilvra was sent to Washington, where he passed two winters in an effort to procure a restoration of those lands to the public domain in the interests of settlers.

He appeared before each committee of the Senate and House to which the various bills introduced upon this subject were referred, and made oral arguments and submitted printed briefs, and finally succeeded in restoring to settlement those lands, amounting to upward of 5,000,000 acres (20,000 km2).

As City Attorney, Judge McGilvra made the application and argued the case before the Register and Receiver of the United States General Land Office at Olympia.

Subsequently, and after the expiration of Judge McGilvra's term of office, the case was complicated by the intervention of other parties claiming the right to locate the land with Valentine scrip.

[3] Among McGilvra's business partners in his law firms were James McNaught, who served in the 1890s as attorney for the Northern Pacific Railroad Company; and his son-in-law Judge Thomas Burke.

In the early 1890s he was opposed to Seattle's then proposed system of parks and boulevards, which he saw as a massive government subsidy to real estate men.

[3] To this day, Madison Street remains the only direct route from Lake Washington to Elliott Bay in Downtown Seattle.