When Illinois was admitted as a State into the Union, McLean was elected to the Fifteenth Congress and served from December 3, 1818, to March 3, 1819.

In 1824, McLean was elected to the United States Senate to fill the vacancy created by the resignation of Ninian Edwards and served from November 23, 1824 to March 3, 1825.

He was unsuccessful in a bid for re-election, not managing to get more than 12 votes of the 27 needed in the legislature through 10 ballots,[1] but resumed the practice of law.

He was again elected to the United States Senate and served from March 4, 1829, until his death, aged 39, in Shawneetown, Illinois in 1830.

[2] This article incorporates public domain material from the Biographical Directory of the United States Congress