McGill's personal and family papers begin in 1794 with a few documents of the Charles Martin family and end in 1931 with letters written to Mrs. Mary McGill, the governor's widow. The collection, which consists primarily of incoming correspondence, deals largely with McGill's public service and political careers (1863-1905). McGill corresponded with individuals such as Mr. and Mrs. William Windom, Horace Austin, Pennock Pusey, Ignatius Donnelly, Henry A. Castle, Carl Gutherz, and other political figures in Minnesota.

In the 1880s, the poltical correspondence is at its most voluminous, beginning with a few letters from minor party officials promising their support to McGill. By that time McGill had been private secretary to Governor Horace Austin and state insurance commissioner since 1873. In 1881 McGill was one of six candidates nominated at the Republican State Convention. Two especially interesting letters are from William Windom and Knute Nelson. Windom stated that he would be happy to support McGill but did not want the fact publicized. Although Windom did not elaborate on this statement, he claimed that the opposition to him in Minnesota was due, in some part, to his friendship with McGill. Nelson's letter is of a similar nature. He was interested in McGill's candidacy but stated that he would not take any part in the campaign. He mentioned no names but warned McGill that he (Nelson) felt that some of McGill's supporters were using McGill as a front for a dark horse candidate of their own. Nelson then relates a similar experience from early in his own political career. Although Lucius F. Hubbard won the nomination, McGill was among the top three nominees on the ballot. In 1883 McGill was reappointed as state insurance commissioner.

In 1886 McGill was again a candidate at the Republican State Convention. Correspondence from around the state pertains to the amount of delegate support McGill could expect from various counties. Because of the agrarian discontent and the rise of the Granger and Farmers' Alliance movements, there was a concerted effort in the Minnesota Republican Party to win over the farmer and labor groups. A letter from C. B. Tyler explains the strong Alliance sentiment in his area. There are a number of letters from Elmer E. Adams, a prominent Fergus Falls Republican leader and newspaper editor who enthusiastically supported McGill and promised the support of the Ottertail County delegation. McGill won the nomination and received congratulatory letters from noteable individuals such as Ignatius Donnelly, Archbishop John Ireland, and T. C. Hodgson.

From 1887 to 1905 the political correspondence has a much more general character, including a spate of correspondence pertaining to the unsuccessful 1887-1888 campaign for re-election. Some letters discuss the Stillwater prison investigation and Washburn's candidacy for the United States Senate. Of note are two letters from William McKinley (1894), while he was still governor of Ohio, concerning his proposed visit to the Twin Cities and his vigorous campaign. McGill supported the presidential candidacy of Cushman K. Davis (1896) and corresponds with both Davis and Henry A. Castle about economics, politics, and foreign affairs. A letter from Henry A. Castle, then auditor in the United States Post Office Department, tells of the general preparations for the Spanish-American War. He states, among other things, that regular army officers were being used to drill and discipline the troops in an effort to avoid the disorganization that had been so common during the Civil War.

McGill kept some correspondence written by Horace Austin, for whom he had served as private secretary (1870-1874). In 1876 Austin describes his position as Third Auditor in the Treasury Department, politics in general, the scandals of the Grant administration, and the election of 1876. In 1903 and 1904 Austin is living in California and enthusiastically writes to McGill about a West Indian citrus fruit growing enterprise in which he was interested. He compared in detail the economic and climatic disadvantages of citrus fruit growing in California against more favorable circumstances in the West Indies. Austin asked McGill to contact individuals in the Twin Cities to form and operate a West Indian fruit growing company. Fred B. Snyder writes to McGill with an unfavorable opinion on the scheme.

William Windom corresponded with McGill sporadically throughout a ten year period. They discuss the Minneapolis Mortgage Loan and Trust Company and real estate and financial interests in Minnesota. In 1882 Windom writes about his campaign and asks McGill to make arrangements for a campaign headquarters in St. Paul's Merchants Hotel. After her husband's death, Mrs. Ellen Windom sought McGill's advice on settling financial affairs.

Interspersed throughout the chronological files are various business and legal papers. Because of a lack of outgoing correspondence, it's difficult to discern McGill's part in some of the transactions. Correspondents and topics include William Eliot Furness of Chicago, on loans made to improve Mankato's streets (1872); letters from other states' real estate and insurance companies, explaining their methods of operation (1883); real estate ventures in Dakota Territory (1883); sale of lots in the South St. Paul stockyards (1887); Charles Shandrew, state insurance commissioner from 1886 to1889, on insurance matters in other states (1890).

Other family and personal correspondence includes letters from James C. Wilson (1893-1905), Mrs. N. C. Wheeler (1895-1905), and James Phillips (1877-1895). The letters from James Wilson, father of Mary E. McGill, provide excellent descriptions of family matters and social and political activities. Correspondence from James M. Phillips of West Grove, Pennsylvania (1877-1894) encompasses land values and transactions. McGill was named administrator of the estate and guardian of the children of John B. Phillips, of St. Paul, who died intestate. McGill was given power of attorney to handle Phillips' family legal matters in Minnesota and Pennsylvania. A letter from James Phillips (1879) provides information on the founding of the town of Faxon, Sibley County, Minnesota Territory in 1857 by John Phillips, Charles Gilfillan, and Alexander Ramsey. The town was wiped out in the panic of 1857.

From 1888 through 1905 many bills, receipts, and other household accounts are found. Expenditures relating to the construction of the McGill home in St. Anthony Park include lists of wages paid to workmen (1902) and the installation of electric and gas fixtures (1904). Other items of interest are letters written to Andrew McGill while he was recuperating from an undisclosed illness in Meadville, Pennsylvania (1896-1897); letters from Charles McGill, at the time state representative from the 28th district; and from Wilson McGill, at the time 12 years old, who writes about his life in St. Paul.

Reserve items (1794-1846) were removed from the chronological correspondence and are chiefly early papers of the Martin family. Three items from Charles Martin, dated 1794-1797, give information on the pay and clothing of soldiers stationed at Fort le Boeuf (Pennsylvania). Documents from 1797 through 1846 concern Martin's two sons, Charles, Jr. and Armand and include an Alleghany County land patent (1979); commissions in the Pennsylvania Militia (1811-1812); family matters; descriptions of the country around Lexington, Kentucky (1818); and pioneer settlements in Missouri (1846).