He was leader of the Gorman-Rasin organization with Isaac Freeman Rasin that controlled the Maryland Democratic Party from the late 1870s until his death in 1906.

[6] Gorman's paternal grandfather, John, emigrated to the U.S. from Ireland circa 1794, first settling in Harrisburg, Pennsylvania before moving to the Baltimore area.

[6][8][9] Gorman attended Howard County public schools[10] and for at least one year his father hired a tutor to teach him and neighboring students.

[11]: 5 In 1850, Peter Gorman used his connections to Maryland Congressmen William T. Hamilton and Edward Hammond to arrange for 11-year old Arthur to serve as a U.S. Senate page.

[6] Some sources state that Gorman accompanied Douglas during his debates with Abraham Lincoln in 1858,[7] although biographer John R. Lambert questions these accounts.

[12] Gorman's experience in the Senate gave him extensive knowledge of parliamentary procedures that he would put to use during his political career.

In September 1866, Republicans who held the senate majority removed him as postmaster because he supported President Andrew Johnson's Reconstruction policies.

[6] At the age of 20 in 1859, Gorman was one of the founding members of the Washington Nationals, the first fully professional baseball team in America.

[14] According to contemporary accounts, Gorman was the team's standout left fielder, but often substituted or filled in at every other position, including pitcher and catcher.

[15] In 1867, he led the Nationals in their first trip over the mountains, in which they beat every midwest team except Rockford, Illinois, which had Albert Spalding as its pitcher.

In February 1903, Gorman and his son-in-law Wilton Lambert attempted but failed to buy the Washington Senators baseball team.

Gorman was closely aligned with Baltimore political leader Isaac Freeman Rasin and supported William Pinkney Whyte for Governor in 1871.

The New York Times reported that the previous legislative election was influenced by large groups of "ward rounders" who shot and wounded black Republican voters at the Howard County polls.

"[27] In his final years, Gorman "spearheaded an attempt by Democrats to disenfranchise black voters in Maryland, who tended to vote Republican."

Related legislation passed easily in the Democratically controlled Senate of early 1904, though Governor Warfield did not sign the bill into law, and it was rejected by voters in late 1905.

The Gormans had five daughters and one son: Ada, Haddie, Grace, Anne Elizabeth ("Bessie"), Mary and Arthur P. Jr.[6] In 1890, Gorman's wife and daughter Grace escaped a fire at their Laurel house "Fairview"; a new Queen Anne style house was built in its place the following year.

[33] Gorman's daughter Ada married Charles Joseph Magness, a young man about half her age, against her family's wishes in 1908.

[37] Gorman's daughter Grace (better known as Daisy) married Richard Alward Johnson, the first manager of the Laurel race track and later a Maryland State Senator, in 1895.

[42][43][44] Gorman's only son, Arthur Jr., attended Lawrenceville Prep and played on the Maryland Agricultural College football team in 1892 and 1893 as a fullback.

[56] An attorney, Lambert helped Arthur Gorman attempt to buy the Washington Senators baseball team in February 1903.

[59] Their son, Ralph Gorman Hills, won a bronze medal for shot put at the 1924 Summer Olympics.

[26][67] He was interred at Oak Hill Cemetery in Washington, D.C.[68][69] Gorman's wife became ill soon after his death, and died on June 21, 1910.