Born on his grandfather's plantation, "Sylvia's Plain," in Washington County, Pennsylvania, near Wheeling, Virginia (now West Virginia), Leffler attended the public schools and was graduated from Jefferson College, (now Washington & Jefferson College), in Canonsburg, Pennsylvania.
In 1826, Leffler was elected as an Adams Party candidate to the Twentieth Congress, defeating incumbent Jacksonian Joseph Johnson.
While under Michigan's regional governance, he was named as the chief justice of the first judicial tribunal of Des Moines County on April 11, 1836.
President John Tyler appointed Leffler as United States marshal for the district of Iowa on December 18, 1843.
He served until removed by President James K. Polk on December 29, 1845, when he resumed the practice of law in Burlington.