Which US president served in Congress after leaving the presidency?

John Quincy Adams. He served in the U.S. House of Representatives for the 8th, 11th, and 12th Massachusetts Congressional districts, depending on the year. He was elected to the office first in the 1830 election and took office on March 4, 1831, just two years after leaving the Presidency. He was a congressman until his death on February 23, 1848, at the ripe old age of 80 years.

It must have been strange for, say, the Speaker of the House to refer to most congressmen as congressmen and refer to JQA as "Mr. President."