He played in Major League Baseball for the Washington Senators, Boston Red Sox, New York Yankees, and Brooklyn Robins between 1913 and 1923.

After retiring from baseball, Harper entered politics, and won election as sheriff of Bergen County as a Republican.

He served in the cabinets of Governors Walter Evans Edge and Alfred E. Driscoll, as the Commissioner of Labor and Industry.

Harper's father died when he was 13 years old, and he dropped out of school after the eighth grade to work in a factory in New York City, earning $7 per week ($237 in current dollar terms).

[1] When the Senators needed to reduce their roster size by a mid-May deadline in 1915, they sent Harper to the Minneapolis Millers of the Class AA American Association.

His most productive season came in 1916 with Washington, when he posted career numbers in wins (14), strikeouts (149) and innings pitched (249+1⁄3), while recording a 2.45 ERA.

[17] After the 1920 season, the Red Sox traded Harper, Waite Hoyt, Mike McNally, and Wally Schang to the New York Yankees for Del Pratt, Muddy Ruel, Hank Thormahlen, and Sammy Vick.

The Yankees won the American League pennant and Harper started Game 6 of the 1921 World Series, but did not earn a decision.

[1][28] The company expanded into construction,[29] helped build the Holland Tunnel and moved pipe that was laid from the Catskill Mountains to New York City.

Harper lost the general election to William H. J. Ely, a Bergen County District Judge, 52%–48%, during what was a difficult year for Republicans due to the unpopularity of President Herbert Hoover's economic policies.

He remained in that role for a decade, until Governor Walter Evans Edge appointed him Commissioner of Labor and Industry in 1944.

[31] In 1948, Harper ran for United States Senate seat occupied by Albert W. Hawkes, who was retiring, against Driscoll's wishes.

Though Harper had the support of the Bergen County Republicans, he narrowly lost the primary election to Widnall, 50%–40%, a margin of 402 votes.

[31][35] Throughout his baseball career, Harper refused to play on Sundays; he held out from the Red Sox after his trade there in part to ensure that clause was added to his contract.