It is not unusual for someone to be elected president after serving in another office. For example, there are lots of presidents of the United States who served as vice president before becoming president. Some of them became president because the original president died, and other people became president because they had the experience necessary to win an election. In some cases, both descriptors apply.
For example, John Adams served as vice president of George Washington. John Adams would eventually become president himself after winning the election in 1796. Thomas Jefferson also served as vice president of John Adams while he was in office. Thomas Jefferson would go on to be elected president after winning the election in 1800. John Tyler was the very first vice president to become president because the original president passed away. William Henry Harrison served as president in 1841, but he died after only being in office for one month. After he passed away, John Tyler would go on to become president.
There are plenty of presidents who have also served as president after serving in the cabinet of another president. For example, Thomas Jefferson also served as the Secretary of State under George Washington. He served in that capacity from 1790 to 1793. James Madison also served as the Secretary of State. He served as Secretary of State under Thomas Jefferson from 1801 to 1809. Some of the other presidents to also serve as Secretary of State include James Monroe, John Quincy Adams, Martin Van Buren, and James Buchanan. The other common position that presidents served in was as Secretary of War. This is a position that is now referred to as Secretary of Defense. James Monroe and William Howard Taft both served as Secretary of War under different presidents. Finally, Herbert Hoover served as Secretary of Commerce. He held this position under Warren G Harding and Calvin Coolidge before he went on to become president himself.
There are also a handful of presidents who have served as ambassadors to other countries. For example, John Adams served as the minister to Britain as a member of the Continental Congress. Thomas Jefferson also served as the minister to France under the Continental Congress. This means that they held that position before the founding of the United States. George Washington would eventually become president of the United States, and James Monroe served as his ambassador to France during that time. James Monroe also served as the minister to Britain under Thomas Jefferson before he went on to be elected president himself. John Quincy Adams served as a minister to a wide variety of places. For example, he served as the minister to the Netherlands under George Washington as well as the minister to Germany under John Adams. He also served as the minister for Russia under James Madison.