Rainey was appointed master in chancery for Greene County, Illinois, from 1887 until 1895, when he resigned, and returned to private practice.

He then decided to return to politics in 1902 getting elected to Congress and serving for nine terms before losing to Guy L. Shaw in 1920.

The 1903 Congressional Directory notes that Rainey "belongs to the Knights of Pythias, the Independent Order of Odd Fellows, the Modern Woodmen, the Mutual Protective League, and the Elks.

Rainey gave the Roosevelt administration carte blanche to do whatever it wanted, allowing almost the entire New Deal to be passed with little or no changes.

Rainey died of a heart attack the following summer, on the eve of his seventy-fourth birthday, before the new Congress could meet.