In 1815, he was urgently solicited to become a candidate for the U.S. Senate, but he declined. He was then but thirty years of age. In 1816, he was unanimously elected judge of the Supreme Court of the state of Ohio and he resigned his seat in Congress. While in Congress he voted for a bill giving to each member a salary of $1,500 a year instead of the per diem allowance. Judge McLean presided on the bench in Ohio for six years, during which time he won for himself an enviable judicial reputation. In 1822, he was appointed commissioner of the general Land Office by President Monroe; and in 1823, he entered the cabinet as Postmaster General. As Postmaster General he secured a fine reputation, improving its finances and in every possible way improving the postal facilities of the country. By an almost unanimous vote of Congress his salary was increased from $4,000 to $6,000. "The distribution of the public patronage of his department exhibited in another respect his qualities as an executive officer, and manifested the rule of action that has always marked his character. The principle upon which executive patronage should be distributed, has been one of the most important questions in this government, and has presented the widest variation between the profession and practice of individuals and parties. In the administration of the post-office department by Judge McLean, an example was presented in strict consistence with sound principles of republican government, and just party organization. During the whole time that the affairs of the department were administered by the judge, he had necessarily a difficult part to act. The country was divided into two great parties, animated by the most determined spirit of rivalry, and each bent on advancing itself to the lead of public affairs. A question was now started, whether it was proper to make political opinions the test of qualification for office. Such a principle had been occasionally acted upon during the preceding periods of our history; but so rarely, as to constitute the exception, rather than the rule. It had never become the settled and systematic course of conduct of any public officer. Doubtless every one is bound to concede something to the temper and opinions of the party to which he belongs, otherwise party would be an association without any connecting bond of alliance.