You might want to check out the "corrupt bargain" of 1824 Presidential election. The race consisted of Henry Clay, John Quincy Adams, and Andrew Jackson. None of the presidents got a majority electoral, thus the vote was given to the House of Representatives to decide. Guess who was the Speaker of the House? Henry Clay. Adams managed to convince Clay to put him into presidential position, and he would in turn give the position of Secretary of State to Clay. This angered Jackson greatly. Adams actually held the position of Secretary of State during both of James Monroe's terms.
Kind of surprised this was not mentioned. Wendell Willkie was the Republican dark horse nominee for the 1940 presidential election. He was defeated in a landslide against President Franklin Roosevelt. Despite this, Roosevelt found common ground with Willkie and appointed him to an "Ambassador-at-large" position. He would represent the President to the Soviets, the British and in the Middle East and China.
He was briefly in the 1944 presidential primaries before dropping out. He died soon after of a heart attack.
Within the twenty years a president appointed his bitterly fought primary opponent to secretary of state.